<PAGE>   1
                       SECURITIES AND EXCHANGE COMMISSION
                              WASHINGTON, DC 20549

                                    FORM 8-K

                             CURRENT REPORT PURSUANT
                          TO SECTION 13 OR 15(D) OF THE
                         SECURITIES EXCHANGE ACT OF 1934

         Date of report (Date of earliest event reported) April 20, 1998

                                  VIASOFT, INC.
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             (Exact Name of Registrant as Specified in Its Charter)

                                    Delaware
                 (State or Other Jurisdiction of Incorporation)


         0-25472                                           94-2892506
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(Commission File Number)                       (IRS Employer Identification No.)

3033 North 44th Street, Phoenix, Arizona                      85018
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(Address of Principal Executive Offices)                     (Zip Code)


                                 (602) 952-0050
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              (Registrant's Telephone Number, Including Area Code)
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ITEM 5. OTHER EVENTS.

         On April 20, 1998, the Board of Directors of Viasoft, Inc. (the
"Company") declared a dividend of one preferred share purchase right (a "Right")
for each outstanding share of common stock, par value $0.001 per share (the
"Common Shares"), outstanding on May 8, 1998 (the "Record Date") to the
stockholders of record on that date. Each Right entitles the registered holder
to purchase from the Company one one-thousandth of a share of Series A Junior
Participating Preferred Stock, par value $0.001 per share (the "Preferred
Shares"), of the Company, at a price of $180.00 per one one-thousandth of a
Preferred Share (the "Purchase Price"), subject to adjustment. The description
and terms of the Rights are set forth in a Rights Agreement (the "Rights
Agreement") between the Company and Harris Trust and Savings Bank, as Rights
Agent (the "Rights Agent").

         Until the earlier to occur of (i) 10 days following a public
announcement that a person or group of affiliated or associated persons (an
"Acquiring Person") has acquired beneficial ownership of 15% or more of the
outstanding Common Shares or (ii) 10 business days (or such later date as may be
determined by action of the Board of Directors prior to such time as any Person
becomes an Acquiring Person) following the commencement of, or announcement of
an intention to make, a tender offer or exchange offer the consummation of which
would result in the beneficial ownership by a person or group of 15% or more of
such outstanding Common Shares (the earlier of such dates being called the
"Distribution Date"), the Rights will be evidenced, with respect to any of the
Common Share certificates outstanding as of the Record Date, by such Common
Share certificate with a copy of the Summary of Rights attached thereto.

         The Rights Agreement provides that, until the Distribution Date (or
earlier redemption or expiration of the Rights), the Rights will be transferred
with and only with the Common Shares. Until the Distribution Date (or earlier
redemption or expiration of the Rights), new Common Share certificates issued
after the Record Date or upon transfer or new issuance of Common Shares will
contain a notation incorporating the Rights Agreement by reference. Until the
Distribution Date (or earlier redemption or expiration of the Rights), the
surrender for transfer of any certificates for Common Shares outstanding as of
the Record Date, even without such notation or a copy of the Summary of Rights
being attached thereto, will also constitute the transfer of the Rights
associated with the Common Shares represented by such certificate. As soon as
practicable following the Distribution Date, separate certificates evidencing
the Rights ("Right Certificates") will be mailed to holders of record of the
Common Shares as of the Close of Business on the Distribution Date and such
separate Right Certificates alone will evidence the Rights.

         The Rights are not exercisable until the Distribution Date. The Rights
will expire on April 20, 2008 (the "Final Expiration Date"), unless the Final
Expiration Date is extended or unless the Rights are earlier redeemed by the
Company, in each case, as described below.

         The Purchase Price payable, and the number of one-thousandths of a
Preferred Share or other securities or property issuable, upon exercise of the
Rights are subject to adjustment from time to time to prevent dilution (i) in
the event of a stock dividend on, or a subdivision, combination or
reclassification of, the Preferred Shares, (ii) upon the grant to holders of the
Preferred Shares of certain rights or warrants to subscribe for or purchase
Preferred Shares at a price, or securities convertible into Preferred Shares
with a conversion price, less than the then current market price of the
Preferred Shares or (iii) upon the distribution to holders of the Preferred
Shares of evidences of indebtedness or assets (excluding regular periodic cash
dividends paid out of earnings or retained earnings or dividends payable in
Preferred Shares) or of subscription rights or warrants (other than those
referred to above).
<PAGE>   3
         The number of outstanding Rights and the number of one-thousandths of a
Preferred Share issuable upon exercise of each Right are also subject to
adjustment in the event of a stock split of the Common Shares or a stock
dividend on the Common Shares payable in Common Shares or subdivisions,
consolidations or combinations of the Common Shares occurring, in any such case,
prior to the Distribution Date.

         Preferred Shares purchasable upon exercise of the Rights will not be
redeemable. Each Preferred Share will be entitled to a quarterly dividend
payment of 1000 times the dividend declared per Common Share. In the event of
liquidation, the holders of the Preferred Shares will be entitled to an
aggregate payment of 1000 times the aggregate payment made per Common Share.
Each Preferred Share will have 1000 votes, voting together with the Common
Shares. In the event of any merger, consolidation or other transaction in which
Common Shares are exchanged, each Preferred Share will be entitled to receive
1000 times the amount received per Common Share. These rights are protected by
customary antidilution provisions.

         Because of the nature of the Preferred Shares' dividend, liquidation
and voting rights, the value of the one one-thousandth interest in a Preferred
Share purchasable upon exercise of each Right should approximate the value of
one Common Share.

         From and after the occurrence of an event whereby a Person becomes an
Acquiring Person, if the Rights evidenced by this Right Certificate are or were
at any time on or after the earlier of (x) the date of such event and (y) the
Distribution Date (as such term is defined in the Rights Agreement) acquired or
beneficially owned by an Acquiring Person or an Associate or Affiliate of an
Acquiring Person (as such terms are defined in the Rights Agreement), such
Rights shall become void, and any holder of such Rights shall thereafter have no
right to exercise such Rights.

         In the event that, at any time after a Person becomes an Acquiring
Person, the Company is acquired in a merger or other business combination
transaction or 50% or more of its consolidated assets or earning power are sold,
proper provision will be made so that each holder of a Right will thereafter
have the right to receive, upon the exercise thereof at the then current
exercise price of the Right, that number of shares of common stock of the
acquiring company which at the time of such transaction will have a market value
of two times the exercise price of the Right. In the event that any person
becomes an Acquiring Person, proper provision shall be made so that each holder
of a Right, other than Rights beneficially owned by the Acquiring Person and its
Affiliates and Associates (which will thereafter be void), will thereafter have
the right to receive upon exercise that number of Common Shares having a market
value of two times the exercise price of the Right. If the Company does not have
sufficient Common Shares to satisfy such obligation to issue Common Shares, or
if the Board of Directors so elects, the Company shall deliver upon payment of
the exercise price of a Right an amount of cash or securities equivalent in
value to the Common Shares issuable upon exercise of a Right; provided that, if
the Company fails to meet such obligation within 30 days following the later of
(x) the first occurrence of an event triggering the right to purchase Common
Shares and (y) the date on which the Company's right to redeem the Rights
expires, the Company must deliver, upon exercise of a Right but without
requiring payment of the exercise price then in effect, Common Shares (to the
extent available) and cash equal in value to the difference between the value of
the Common Shares otherwise issuable upon the exercise of a Right and the
exercise price then in effect. The Board of Directors may extend the 30-day
period described above for up to an additional 60 days to permit the taking of
action that may be necessary to authorize sufficient additional Common Shares to
permit the issuance of Common Shares upon the exercise in full of the Rights.
<PAGE>   4
         At any time after any Person becomes an Acquiring Person and prior to
the acquisition by any person or group of a majority of the outstanding Common
Shares, the Board of Directors of the Company may exchange the Rights (other
than Rights owned by such person or group which have become void), in whole or
in part, at an exchange ratio of one Common Share per Right (subject to
adjustment).

         With certain exceptions, no adjustment in the Purchase Price will be
required until cumulative adjustments require an adjustment of at least 1% in
such Purchase Price. No fractional Preferred Shares will be issued (other than
fractions which are integral multiples of one one-thousandth of a Preferred
Share, which may, at the election of the Company, be evidenced by depositary
receipts) and in lieu thereof, an adjustment in cash will be made based on the
market price of the Preferred Shares on the last trading day prior to the date
of exercise.

         At any time prior to the time any Person becomes an Acquiring Person,
the Board of Directors of the Company may redeem the Rights in whole, but not in
part, at a price of $0.01 per Right (the "Redemption Price"). The redemption of
the Rights may be made effective at such time, on such basis and with such
conditions as the Board of Directors in its sole discretion may establish.
Immediately upon any redemption of the Rights, the right to exercise the Rights
will terminate and the only right of the holders of Rights will be to receive
the Redemption Price.

         The terms of the Rights may be amended by the Board of Directors of the
Company without the consent of the holders of the Rights, except that from and
after such time as any person becomes an Acquiring Person no such amendment may
adversely affect the interests of the holders of the Rights (other than the
Acquiring Person and its Affiliates and Associates).

         Until a Right is exercised, the holder thereof, as such, will have no
rights as a stockholder of the Company, including, without limitation, the right
to vote or to receive dividends.

         This summary description of the Rights does not purport to be complete
and is qualified in its entirety by reference to the Agreement, which is hereby
incorporated herein by reference and filed herewith as Exhibit 4.


ITEM 7. FINANCIAL STATEMENTS AND EXHIBITS.

<TABLE>
<CAPTION>
Exhibit
Number            Description
------            -----------
<S>               <C>
4                 Rights Agreement, dated as of April 20, 1998 between the
                  Company and Harris Trust and Savings Bank, including the
                  Certificate of Designation of Series A Junior Participating
                  Preferred Stock, Form of Right Certificate and Summary of
                  Rights to Purchase Preferred Shares, attached thereto as
                  Exhibits A, B and C respectively.
</TABLE>
<PAGE>   5
                                   SIGNATURES

         Pursuant to the requirements of the Securities Exchange Act of 1934,
the registrant has duly caused this report to be signed on its behalf by the
undersigned, hereunto duly authorized.

                                        VIASOFT, INC.


         Date: April 22, 1998           By:  /s/ Catherine R. Hardwick
                                             -------------------------
                                             Catherine R. Hardwick
                                             Vice President and General Counsel

<PAGE>   6
                                  EXHIBIT INDEX


<TABLE>
<CAPTION>
Exhibit  No.      Exhibit
------------      -------
<S>               <C>
4                 Rights Agreement, dated as of April 20, 1998 between the
                  Company and Harris Trust and Savings Bank, including the
                  Certificate of Designation of Series A Junior Participating
                  Preferred Stock, Form of Right Certificate and Summary of
                  Rights to Purchase Preferred Shares, attached thereto as
                  Exhibits A, B and C respectively.
</TABLE>