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                                                                  EXHIBIT 10.33

                                                                  Execution Copy
                                                                  --------------

                            REGISTRATION AGREEMENT
                            ----------------------


     THIS REGISTRATION AGREEMENT (this "Agreement") is made as of March 23,
                                        ---------
1999 by and among (i) ZC Acquisition Corp., a Delaware corporation (the
"Company"), (ii) GTCR Fund VI, L.P., a Delaware limited partnership ("GTCR Fund
 -------                                                              ---------
VI"), GTCR VI Executive Fund, L.P., a Delaware limited partnership ("Executive
--                                                                   ---------
Fund") and GTCR Associates VI, a Delaware general partnership ("Associates
----                                                            ----------
Fund") (each an "Investor" and collectively, the "Investors"), (iii) William
                 --------                         ---------
Seibel and any other executive employee of the Company who, at any time,
acquires securities of the Company in accordance with Section 8 hereof and
                                                      ---------
executes a counterpart of this Agreement or otherwise agrees to be bound by this
Agreement (each, an "Executive" and collectively, the "Executives"), and (iv)
                     ---------                         ----------
each of the other entities and individuals set forth from time to time on the
attached "Schedule of Stockholders" under the heading "Other Stockholders" who,
          ------------------------                     ------------------
at any time, acquires securities of the Company in accordance with Section 8
                                                                   ---------
hereof and executes a counterpart of this Agreement or otherwise agrees to be
bound by this Agreement.

     The Company and the Investors are parties to a Purchase Agreement of even
date herewith (the "Purchase Agreement"). In order to induce the Investors to
                    ------------------
enter into the Purchase Agreement, the Company has agreed to provide the
registration rights set forth in this Agreement. The execution and delivery of
this Agreement is a condition to the Closing under the Purchase Agreement.
Unless otherwise provided in this Agreement, capitalized terms used herein shall
have the meanings set forth in Section 9 hereof.
                               ---------

     The parties hereto agree as follows:

     1.   Demand Registrations.
          --------------------

          (a)  Requests for Registration. At any time, the holders of a
               -------------------------
majority of the Investor Registrable Securities may request registration under
the Securities Act of all or any portion of their Registrable Securities having
an aggregate offering price of at least $5,000,000 (based on the then current
market price or fair value at the time of such request) on Form S- 1 or any
similar long-form registration ("Long-Form Registrations"), or on Form S-2 or S-
                                 -----------------------
3 or any similar short-form registration ("Short-Form Registrations") if
                                           ------------------------
available.  All registrations requested pursuant to this Section 1(a) are
                                                         ------------
referred to herein as "Demand Registrations".  Each request for a Demand
                       --------------------
Registration shall specify the approximate number of Registrable Securities
requested to be registered and the anticipated per share price range for such
offering.  Within ten days after receipt of any such request, the Company shall
give written notice of such requested registration to all other holders of
Registrable Securities and shall include in such registration all Registrable
Securities with respect to which the Company has received written requests for
inclusion therein within 15 days after the receipt of the Company's notice.

          (b)  Investor Long-Form Registrations.  The holders of Investor
               --------------------------------
Registrable Securities shall be entitled to request an unlimited number of Long-
Form Registrations in which the Company shall pay all Registration Expenses (as
defined in Section 5) ("Company-paid Long-Form Registrations").  All Long-Form
           ---------    ------------------------------------
Registrations shall be underwritten registrations.
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          (c)  Investor Short-Form Registrations.  In addition to the Long-Form
               ---------------------------------
Registrations provided pursuant to Section 1 (b), the holders of Investor
                                   ----------
Registrable Securities shall be entitled to request an unlimited number of
Short-Form Registrations in which the Company shall pay all Registration
Expenses.  Demand Registrations shall be Short-Form Registrations whenever the
Company is permitted to use any applicable short form.  After the Company has
become subject to the reporting requirements of the Securities Exchange Act, the
Company shall use its best efforts to make Short-Form Registrations on Form S-3
available for the sale of Registrable Securities.

          (d)  Priority on Demand Registration.  The Company shall not include
               -------------------------------
in any Demand Registration any securities which are not Registrable Securities
without the prior written consent of the holders of a majority of the
Registrable Securities included in such registration. If a Demand Registration
is an underwritten offering and the managing underwriters advise the Company in
writing that in their opinion the number of Registrable Securities and, if
permitted hereunder, other securities requested to be included in such offering
exceeds the number of Registrable Securities and other securities, if any, which
can be sold in an orderly manner in such offering within a price range
acceptable to the holders of a majority of the Registrable Securities to be
included in such registration therein, the Company shall include in such
registration prior to the inclusion of any securities which are not Registrable
Securities the number of Registrable Securities requested to be included which
in the opinion of such underwriters can be sold in an orderly manner within the
price range of such offering, pro rata among the respective holders thereof on
the basis of the amount of Registrable Securities owned by each such holder.

          (e)  Restrictions on Long-Form Registrations. The Company shall not be
               ---------------------------------------
obligated to effect any Long-Form Registration within 90 days after the
effective date of a previous Long-Form Registration or a previous registration
in which the holders of Registrable Securities were given piggyback rights
pursuant to Section 2 and in which there was no reduction in the number of
            ---------
Registrable Securities requested to be included.  The Company may postpone for
up to 180 days the filing or the effectiveness of a registration statement for a
Demand Registration if the Company and the holders of a majority of the
Registrable Securities agree that such Demand Registration would reasonably be
expected to have a material adverse effect on any proposal or plan by the
Company or any of its Subsidiaries to engage in any acquisition of assets (other
than in the ordinary course of business) or any merger, consolidation, tender
offer, reorganization or similar transaction; provided however, that in such
                                              -------- -------
event, the holders of Registrable Securities initially requesting such Demand
Registration shall be entitled to withdraw such request and the Company shall
pay all Registration Expenses in connection with such registration.  The Company
may delay a Demand Registration hereunder only once in any twelve-month period.

          (f)  Selection of Underwriters.  The holders of a majority of the
               -------------------------
Registrable Securities included in any Demand Registration shall have the right
to select the investment banker(s) and manager(s) to administer the offering.

          (g)  Other Registration Rights.  Except as provided in this Agreement,
               -------------------------
the Company shall not grant to any Persons the right to request the Company to
register any equity securities of the Company, or any securities convertible or
exchangeable into or exercisable for

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such securities, without the prior written consent of the holders of a majority
of the Registrable Securities.

     2.   Piggyback Registrations.
          -----------------------

          (a)  Right to Piggyback. Whenever the Company proposes to register any
               ------------------
of its securities under the Securities Act (other than (i) pursuant to a Demand
Registration, (ii) in connection with an initial public offering of the
Company's equity securities, or (iii) in connection with registrations on form
S-8 or any successor form) and the registration form to be used may be used for
the registration of Registrable Securities (a "Piggyback Registration"), the
                                               ----------------------
Company shall give prompt written notice to all holders of Registrable
Securities of its intention to effect such a registration and shall include in
such registration all Registrable Securities with respect to which the Company
has received written requests for inclusion therein within 20 days after the
receipt of the Company's notice.

          (b)  Piggyback Expenses.  The Registration Expenses of the holders of
               ------------------
Registrable Securities shall be paid by the Company in all Piggyback
Registrations.

          (c)  Priority on Primary Registrations. If a Piggyback Registration is
               ---------------------------------
an underwritten primary registration on behalf of the Company, and the managing
underwriters advise the Company in writing that in their opinion the number of
securities requested to be included in such registration exceeds the number
which can be sold in an orderly manner in such offering within a price range
acceptable to the Company, the Company shall include in such registration (i)
first, the securities the Company proposes to sell, (ii) second, the Registrable
Securities requested to be included in such registration, pro rata among the
holders of such Registrable Securities on the basis of the number of shares
owned by each such holder, and (iii) third, other securities requested to be
included in such registration.

          (d)  Priority on Secondary Registrations.  If a Piggyback Registration
               -----------------------------------
is an underwritten secondary registration on behalf of holders of the Company's
securities (other than holders of Registrable Securities; it being understood
that secondary registrations on behalf of holders of Registrable Securities are
addressed in Section I above rather than this Section 2(d)), and the managing
             ---------
underwriters advise the Company in writing that in their opinion the number of
securities requested to be included in such registration exceeds the number
which can be sold in an orderly manner in such offering within a price range
acceptable to the holders of a majority of the Registrable Securities to be
included in such registration, the Company shall include in such registration
(i) first, the securities requested to be included therein by the holders
requesting such registration, (ii) second, the Registrable Securities requested
to be included in such registration, pro rata among the holders of such
Registrable Securities on the basis of the number of shares owned by each such
holder, and (iii) third, other securities requested to be included in such
registration.

          (e)  Selection of Underwriters.  If any Piggyback Registration is an
               -------------------------
underwritten offering, the selection of investment banker(s) and manager(s) for
the offering must be approved by the holders of a majority of the Registrable
Securities included in such Piggyback Registration.  Such approval shall not be
unreasonably withheld.

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          (f)  Other Registrations.  If the Company has previously filed a
               -------------------
registration statement with respect to Registrable Securities pursuant to
Section 1 or pursuant to this Section 2, and if such previous registration has
---------                     ---------
not been withdrawn or abandoned, the Company shall not file or cause to be
effected any other registration of any of its equity securities or securities
convertible or exchangeable into or exercisable for its equity securities under
the Securities Act (except on Form S-8 or any successor form), whether on its
own behalf or at the request of any holder or holders of such securities, until
a period of at least 180 days has elapsed from the effective date of such
previous registration.

     3.   Holdback Agreements,
          -------------------

          (a)  Each holder of Registrable Securities and each officer and
director of the Company shall not effect any public sale or distribution
(including sales pursuant to Rule 144) of equity securities of the Company, or
any securities convertible into or exchangeable or exercisable for such
securities, during the seven days prior to and the 180-day period beginning on
the effective date of any underwritten Demand Registration, any underwritten
Piggyback Registration in which Registrable Securities are included (except as
part of such underwritten registration), or the initial public offering, unless
the underwriters managing the registered public offering otherwise agree.

          (b)  The Company (i) shall not effect any public sale or distribution
of its equity securities, or any securities convertible into or exchangeable or
exercisable for such securities, during the seven days prior to and during the
180-day period beginning on the effective date of any underwritten Demand
Registration or any underwritten Piggyback Registration (except as part of such
underwritten registration or pursuant to registrations on Form S-8 or any
successor form), unless the underwriters managing the registered public offering
otherwise agree, and (ii) shall cause each holder of its Common Stock, or any
securities convertible into or exchangeable or exercisable for Common Stock,
purchased from the Company at any time after the date of this Agreement (other
than in a registered public offering) to agree not to effect any public sale or
distribution (including sales pursuant to Rule 144) of any such securities
during such period (except as part of such underwritten registration, if
otherwise permitted), unless the underwriters managing the registered public
offering otherwise agree. Notwithstanding the foregoing, clause (ii) of this
Section 3(b) shall not apply to any holder of the Company's Common Stock and/or
any securities convertible into or exchangeable or exercisable for Common Stock,
if such Person holds less than 1% of the Company's Common Stock on a fully
diluted basis.

     4.   Registration Procedures.  Whenever the holders of Registrable
          -----------------------
Securities have requested that any Registrable Securities be registered pursuant
to this Agreement, the Company shall use its reasonable best efforts to effect
the registration and the sale of such Registrable Securities in accordance with
the intended method of disposition thereof, and pursuant thereto the Company
shall be as expeditious as possible:

          (a)  prepare and file with the Securities and Exchange Commission a
registration statement with respect to such Registrable Securities and use its
reasonable best efforts to cause such registration statement to become effective
(provided that before filing a registration statement or prospectus or any
amendments or supplements thereto, the Company

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shall furnish to one counsel selected by the holders of a majority of the
Registrable Securities covered by such registration statement copies of all such
documents proposed to be filed, which documents shall be subject to the review
and comment of such counsel);

          (b)  notify each holder of Registrable Securities of the effectiveness
of each registration statement filed hereunder and prepare and file with the
Securities and Exchange Commission such amendments and supplements to such
registration statement and the prospectus used in connection therewith as may be
necessary to keep such registration statement effective for a period of not less
than 180 days and comply with the provisions of the Securities Act with respect
to the disposition of all securities covered by such registration statement
during such period in accordance with the intended methods of disposition by the
sellers thereof set forth in such registration statement;

          (c)  furnish to each seller of Registrable Securities such number of
copies of such registration statement, each amendment and supplement thereto,
the prospectus included in such registration statement (including each
preliminary prospectus) and such other documents as such seller may reasonably
request in order to facilitate the disposition of the Registrable Securities
owned by such seller;

          (d)  use its reasonable best efforts to register or qualify such
Registrable Securities under such other securities or blue sky laws of such
jurisdictions as any seller reasonably requests and do any and all other acts
and things which may be reasonably necessary or advisable to enable such seller
to consummate the disposition in such jurisdictions of the Registrable
Securities owned by such seller (provided that the Company shall not be required
to (i) qualify generally to do business in any jurisdiction where it would not
otherwise be required to qualify but for this subparagraph, (ii) subject itself
to taxation in any such jurisdiction or (iii) consent to general service of
process in any such jurisdiction);

          (e)  notify each seller of such Registrable Securities, at any time
when a prospectus relating thereto is required to be delivered under the
Securities Act, of the happening of any event as a result of which the
prospectus included in such registration statement contains an untrue statement
of a material fact or omits any fact necessary to make the statements therein
not misleading, and, at the request of the holders of a majority of the
Registrable Securities covered by such registration statement, the Company shall
prepare a supplement or amendment to such prospectus so that, as thereafter
delivered to the purchasers of such Registrable Securities, such prospectus
shall not contain an untrue statement of a material fact or omit to state any
fact necessary to make the statements therein not misleading;

          (f)  cause all such Registrable Securities to be listed on each
securities exchange on which similar securities issued by the Company are then
listed and, if not so listed, to be listed on the NASD automated quotation
system and, if listed on the NASD automated quotation system, use its best
efforts to secure designation of all such Registrable Securities covered by such
registration statement as a NASDAQ "national market system security" within the
meaning of Rule 11 Aa2-1 of the Securities and Exchange Commission or, failing
that, to secure NASDAQ authorization for such Registrable Securities;

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          (g)  provide a transfer agent and registrar for all such Registrable
Securities not later than the effective date of such registration statement;

          (h)  enter into such customary agreements (including underwriting
agreements in customary form) and take all such other actions as the holders of
a majority of the Registrable Securities being sold or the underwriters, if any,
reasonably request in order to expedite or facilitate the disposition of such
Registrable Securities (including effecting a stock split or a combination of
shares);

          (i)  otherwise use its commercially reasonable best efforts to comply
with all applicable rules and regulations of the Securities and Exchange
Commission, and make available to its security holders, as soon as reasonably
practicable, an earnings statement covering the period of at least twelve months
beginning with the first day of the Company's first full calendar quarter after
the effective date of the registration statement, which earnings statement shall
satisfy the provisions of Section 11 (a) of the Securities Act and Rule 158
thereunder;

          (j)  in the event of the issuance of any stop order suspending the
effectiveness of a registration statement, or of any order suspending or
preventing the use of any related prospectus or suspending the qualification of
any common stock included in such registration statement for sale in any
jurisdiction, the Company shall use its reasonable best efforts promptly to
obtain the withdrawal of such order;

          (k)  use its commercially reasonable best efforts to cause such
Registrable Securities covered by such registration statement to be registered
with or approved by such other governmental agencies or authorities as may be
necessary to enable the sellers thereof to consummate the -disposition of such
Registrable Securities; and

          (l)  in connection with any underwritten public offering, obtain a
cold comfort letter from the Company's independent public accountants in
customary form and covering such matters of the type customarily covered by cold
comfort letters addressed to the underwriters of such offering.

     5.   Registration Expenses.
          ---------------------

          (a)  All expenses incident to the Company's performance of or
compliance with this Agreement, including without limitation all registration
and filing fees, expenses and fees of compliance with securities or blue sky
laws, printing expenses, messenger and delivery expenses, fees and disbursements
of custodians, and fees and disbursements of counsel for the Company and all
independent certified public accountants, underwriters (excluding discounts and
commissions) and other Persons retained by the Company (all such expenses being
herein called "Registration Expenses"), shall be borne as provided in this
               ---------------------
Agreement, except that the Company shall, in any event, pay its internal
expenses (including, without limitation, all salaries and expenses of its
officers and employees performing legal or accounting duties), the expense of
any annual audit or quarterly review, the expense of any liability insurance and
the expenses and fees for listing the securities to be registered on each
securities exchange on which similar securities issued by the Company are then
listed or on the NASD automated quotation system.

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          (b)  In connection with each Demand Registration and each Piggyback
Registration, the Company shall reimburse the holders of Registrable Securities
included in such registration for the reasonable fees and disbursements of one
counsel chosen by the holders of a majority of the Registrable Securities
included in such registration.

          (c)  To the extent Registration Expenses are not required to be paid
by the Company, each holder of securities included in any registration hereunder
shall pay those Registration Expenses allocable to the registration of such
holder's securities so included, and any Registration Expenses not so allocable
shall be borne by all sellers of securities included in such registration in
proportion to the aggregate selling price of the securities to be so registered.

     6.   Indemnification.
          ---------------

          (a)  The Company agrees to indemnify, to the extent permitted by law,
each holder of Registrable Securities, its officers and directors and each
Person who controls such holder (within the meaning of the Securities Act)
against all losses, claims, damages, liabilities and expenses caused by any
untrue or alleged untrue statement of material fact contained in any
registration statement, prospectus or preliminary prospectus or any amendment
thereof or supplement thereto or any omission or alleged omission of a material
fact required to be stated therein or necessary to make the statements therein
not misleading, except insofar as the same are caused by or contained in any
information furnished in writing to the Company by such holder expressly for use
therein or by such holder's failure to deliver a copy of the registration
statement or prospectus or any amendments or supplements thereto after the
Company has furnished such holder with a sufficient number of copies of the
same. In connection with an underwritten offering, the Company shall indemnify
such underwriters, their officers and directors and each Person who controls
such underwriters (within the meaning of the Securities Act) to the same extent
as provided above with respect to the indemnification of the holders of
Registrable Securities.

          (b)  In connection with any registration statement in which a holder
of Registrable Securities is participating, each such holder will furnish to the
Company in writing such information and affidavits as the Company reasonably
requests for use in connection with any such registration statement or
prospectus and, to the full extent permitted by law, shall indemnify the
Company, its directors and officers and each Person who controls the Company
(within the meaning of the Securities Act) against any losses, claims, damages,
liabilities and expenses resulting from any untrue or alleged untrue statement
of material fact contained in the registration statement, prospectus or
preliminary prospectus or any amendment thereof or supplement thereto or any
omission or alleged omission of a material fact required to be stated therein or
necessary to make the statements therein not misleading, but only to the extent
that such untrue statement or omission is contained in any information or
affidavit so furnished in writing by such holder; provided that the obligation
to indemnify will be individual, not joint and several, for each holder and
shall be limited to the net amount of proceeds received by such holder from the
sale of Registrable Securities pursuant to such registration statement.

          (c)  Any Person entitled to indemnification hereunder will (i) give
prompt written notice to the indemnifying party of any claim with respect to
which it seeks indemnification (provided that the failure to give prompt notice
shall not impair any Person's

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right to indemnification hereunder to the extent such failure has not prejudiced
the indemnifying party) and (ii) unless in such indemnified party's reasonable
judgment a conflict of interest between such indemnified and indemnifying
parties may exist with respect to such claim, permit such indemnifying party to
assume the defense of such claim with counsel reasonably satisfactory to the
indemnified party. If such defense is assumed, the indemnifying party will not
be subject to any liability for any settlement made by the indemnified party
without its consent (but such consent will not be unreasonably withheld). An
indemnifying party who is not entitled to, or elects not to, assume the defense
of a claim will not be obligated to pay the fees and expenses of more than one
counsel for all parties indemnified by such indemnifying party with respect to
such claim, unless in the reasonable judgment of any indemnified party a
conflict of interest may exist between such indemnified party and any other of
such indemnified parties with respect to such claim.

          (d)  The indemnification provided for under this Agreement shall
remain in full force and effect regardless of any investigation made by or on
behalf of the indemnified party or any officer, director or controlling Person
of such indemnified party and shall survive the transfer of securities. The
Company also agrees to make such provisions, as are reasonably requested by any
indemnified party, for contribution to such party in the event the Company's
indemnification is unavailable for that reason.

     7.   Participation in Underwritten Registrations.  No Person may
          -------------------------------------------
participate in any registration hereunder which is underwritten unless such
Person (i) agrees to sell such Person's securities on the basis provided in any
underwriting arrangements approved by the Person or Persons entitled hereunder
to approve such arrangements and (ii) completes and executes all questionnaires,
powers of attorney, indemnities, underwriting agreements and other documents
reasonably required under the terms of such underwriting arrangements; provided
that no holder of Investor Registrable Securities included in any underwritten
registration shall be required to make any representations or warranties to the
Company or the underwriters (other than representations and warranties regarding
such holder, such holder's title to such shares and such holder's intended
method of distribution) or to undertake any indemnification obligations to the
Company or the underwriters with respect thereto, except as otherwise provided
in Section 6 hereof.
   ---------

     8.   Additional Stockholders.  In connection with the issuance of any
          -----------------------
additional equity securities of the Company, the Company, with the consent of
the holders of a majority of the Registrable Securities, may permit such person
to become a party to this Agreement and succeed to all of the rights and
obligations of an "Other Stockholder" under this Agreement by obtaining an
executed counterpart signature page to this Agreement, and, upon such execution,
such person shall for all purposes be an "Other Stockholder" party to this
Agreement.

     9.   Definitions.
          -----------

          (a)  "Executive Registrable Securities" means any shares of Common
                --------------------------------
Stock held as of the date hereof, or acquired hereafter from the Company or
another Stockholder, by the Executives.

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          (b)  "Investor Registrable Securities" means (i) any Common Stock
                -------------------------------
issued to the Investors pursuant to the Purchase Agreement (whether issued
before, on or after the date hereof), (ii) any other Common Stock issued or
issuable with respect to the securities referred to in clause (i) by way of a
stock dividend or stock split or in connection with an exchange or combination
of shares, recapitalization, merger, consolidation or other reorganization, and
(iii) any other shares of Common Stock held by Persons holding securities
described in clauses (i) and (ii), inclusive, above.

          (c)  "Other Stockholder Registrable Securities" means any shares of
                ----------------------------------------
Common Stock held as of the date hereof, or acquired hereafter from the Company,
by the Other Stockholders.

          (d)  "Registrable Securities" means the Investor Registrable
                ----------------------
Securities, Executive Registrable Securities and Other Stockholder Registrable
Securities.  As to any particular Registrable Securities, such securities shall
cease to be Registrable Securities when they have been distributed to the public
pursuant to an offering registered under the Securities Act or sold to the
public through a broker, dealer or market maker in compliance with Rule 144
under the Securities Act (or any similar rule then in force).  For purposes of
this Agreement, a Person shall be deemed to be a holder of Registrable
Securities whenever such Person has the right to acquire such Registrable
Securities (upon conversion or exercise in connection with a transfer of
securities or otherwise, but disregarding any restrictions or limitations upon
the exercise of such right), whether or not such acquisition has actually been
effected.

          (e)  Unless otherwise stated, other capitalized terms contained herein
have the meanings set forth in the Purchase Agreement.

     10.  Miscellaneous.
          -------------

          (a)  No Inconsistent Agreements. The Company shall not hereafter enter
               --------------------------
into any agreement with respect to its securities which is inconsistent with or
violates the rights granted to the holders of Registrable Securities in this
Agreement.

          (b)  Adjustments Affecting Registrable Securities.  The Company shall
               --------------------------------------------
not take any action, or permit any change to occur, with respect to its
securities which would adversely affect the ability of the holders of
Registrable Securities to include such Registrable Securities in a registration
undertaken pursuant to this Agreement or which would adversely affect the
marketability of such Registrable Securities in any such registration
(including, without limitation, effecting a stock split or a combination of
shares).

          (c)  Remedies.  Any Person having rights under any provision of this
               --------
Agreement shall be entitled to enforce such rights specifically to recover
damages caused by reason of any breach of any provision of this Agreement and to
exercise all other rights granted by law.  The parties hereto agree and
acknowledge that money damages may not be an adequate remedy for any breach of
the provisions of this Agreement and that any party may in its sole discretion
apply to any court of law or equity of competent jurisdiction (without posting
any bond or other security) for specific performance and for other injunctive
relief in order to enforce or prevent violation of the provisions of this
Agreement.

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          (d)  Amendments and Waivers. Except as, otherwise provided herein, the
               ----------------------
provisions of this Agreement may be amended or waived only upon the prior
written consent of the Company and holders of at least a majority of the
Registrable Securities.

          (e)  Successors and Assigns.  All covenants and agreements in this
               ----------------------
Agreement by or on behalf of any of the parties hereto shall bind and inure to
the benefit of the respective successors and assigns of the parties hereto
whether so expressed or not. In addition, whether or not any express assignment
has been made, the provisions of this Agreement which are for the benefit of
purchasers or holders of Registrable Securities are also for the benefit of, and
enforceable by, any subsequent holder of Registrable Securities. Notwithstanding
anything in this Section 10(e) to the contrary, the covenants, agreements and
provisions of this Agreement which are for the benefit of purchasers or holders
of Registrable Securities shall not be for the benefit of or enforceable by any
successor, assignee or subsequent holder of the Company's Common Stock if such
Person holds less than 1% of the Company's outstanding shares of Common Stock.

          (f)  Severability. Whenever possible, each provision of this Agreement
               ------------
shall be interpreted in such manner as to be effective and valid under
applicable law, but if any provision of this Agreement is held to be prohibited
by or invalid under applicable law, such provision shall be ineffective only to
the extent of such prohibition or invalidity, without invalidating the remainder
of this Agreement.

          (g)  Counterparts.  This Agreement may be executed simultaneously in
               ------------
two or more counterparts, any one of which need not contain the signatures of
more than one party, but all such counterparts taken together shall constitute
one and the same Agreement.

          (h)  Descriptive Headings.  The descriptive headings of this Agreement
               --------------------
are inserted for convenience only and do not constitute a part of this
Agreement.

          (i)  Governing Law.  The laws of Delaware shall govern all issues
               -------------
concerning the relative rights of the Company and its stockholders and all other
questions concerning the construction, validity and interpretation of this
Agreement, without giving effect to any choice of law or other conflict of law
provision or rule (whether of the State of Delaware or any other jurisdiction)
that would cause the application of the laws of any jurisdiction other than the
State of Delaware.

          (j)  Notices. All notices, demands or other communications to be given
               -------
or delivered under or by reason of the provisions of this Agreement shall be in
writing and shall be deemed to have been given when delivered personally to the
recipient, sent to the recipient by reputable overnight courier service (charges
prepaid) or mailed to the recipient by certified or registered mail, return
receipt requested and postage prepaid.  Such notices, demands and other
communications shall be sent to each Investor and to each Executive at the
addresses indicated on the Schedule of Stockholders and to the Company at the
address of its corporate headquarters or to such other address or to the
attention of such other person as the recipient party has specified by prior
written notice to the sending party.

                                      -10-
<PAGE>

                                                                  EXECUTION COPY
                                                                  --------------

          IN WITNESS WHEREOF, the parties have executed this Registration
Agreement as of the date first written above.

                              ZC ACQUISITION CORP.

                              By:    /s/ William Seibel
                                    ----------------------------------
                              Its:  President

                              GTCR FUND VI, L.P.

                              By:   GTCR Partners VI, L.P.
                              Its:  General Partner

                              By:   GTCR Golder Rauner, L.L.C.
                              Its:  General Partner

                              By:    /s/ Philip A. Canfield
                                    ----------------------------------
                              Name:   Philip A. Canfield
                                    ----------------------------------
                              Its:   Principal

                              GTCR VI EXECUTIVE FUND, L.P.

                              By:   GTCR Partners VI, L.P.
                              Its:  General Partner

                              By:   GTCR Golder Rauner, L.L.C.
                              Its:  General Partner

                              By:   /s/ Philip A. Canfield
                                    ----------------------------------
                              Name:    Philip A. Canfield
                                    ----------------------------------
                              Its:  Principal

                              GTCR ASSOCIATES VI

                              By:   GTCR Partners VI, L.P.
                              Its:  Managing General Partner

                              By:   GTCR Golder Rauner, L.L.C.
                              Its:  General Partner

                              By:    /s/ Philip A. Canfield
                                    ----------------------------------
                              Name:    Philip A. Canfield
                                    ----------------------------------
                              Its:   Principal

                                    /s/ William Seibel
                              ----------------------------------------
                              William Seibel

                  SIGNATURE PAGE TO THE REGISTRATION AGREEMENT

                                      -11-
<PAGE>

                                                                  EXECUTION COPY
                                                                  --------------

                           SCHEDULE OF STOCKHOLDERS
                           ------------------------


GTCR FUND VI, L.P.
6100 Sears Tower
Chicago, IL 60606-6402
Attention: Philip A. Canfield
           Timothy P. McAdam

GTCR VI EXECUTIVE FUND, L.P.
6100 Sears Tower
Chicago, IL 60606-6402
Attention: Philip A. Canfield
           Timothy P. McAdam

GTCR ASSOCIATES VI
6100 Sears Tower
Chicago, IL 60606-6402
Attention: Philip A. Canfield
           Timothy P. McAdam

William Seibel
7 Amherst Road
Wellesley, MA 02181



                                      -12-