<PAGE>   1
                                                                   EXHIBIT 10.45

                          SUBORDINATED PROMISSORY NOTE


$500,000                                                DATE: SEPTEMBER 24, 1999

                                                        DUE:  OCTOBER 1, 2002


FOR VALUE RECEIVED, Telocity, Inc., a California corporation (the "Borrower")
hereby promises to pay to the order of Comdisco, Inc., a Delaware corporation
(the "Lender") at P.O. Box 91744, Chicago, IL 60693 or such other place of
payment as the holder of this Secured Promissory Note (this "Note") may specify
from time to time in writing, in lawful money of the United States of America,
the principal amount of Five Hundred Thousand and 00/100 Dollars ($500,000.00)
together with interest at twelve and one-half percent (12.5%) per annum from
the date of this Note to maturity of each installment on the principal hereof
remaining from time to time unpaid, such principal and interest to be paid in
36 equal monthly installments of $16,767.05 each, commencing November 1, 1999
and on the same day of each month thereafter to and including October 1, 2002,
such installments to be applied first to accrued and unpaid interest and the
balance to unpaid principal. Interest shall be computed on the basis of a year
consisting of twelve months of thirty days each.

The Note is the Note referred to in, and is executed and delivered in
connection with, that certain Subordinated Loan and Security Agreement dated
May 6, 1999 by and between Borrower and Lender (as the same may from time to
time be amended, modified or supplemented in accordance with its terms, the
"Loan Agreement"), and is entitled to the benefit and security of the Loan
Agreement and the other Loan Documents (as defined in the Loan Agreement), to
which reference is made for a statement of all of the terms and conditions
thereof. All terms defined in the Loan Agreement shall have the same
definitions when used herein, unless otherwise defined herein.

THIS NOTE IS EXPRESSLY SUBJECT TO THE TERMS OF THAT CERTAIN SUBORDINATION
AGREEMENT BY AND BETWEEN LENDER AND BORROWER FOR THE BENEFIT OF SENIOR
CREDITOR. IN THE EVENT OF ANY CONTRADICTION OR INCONSISTENCY BETWEEN THIS NOTE
AND THE SUBORDINATION AGREEMENT, THE TERMS OF THE SUBORDINATION AGREEMENT SHALL
CONTROL.

The Borrower waives presentment and demand for payment, notice of dishonor,
protest and notice of protest and any other notice as permitted under the UCC
or any applicable law.



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This Note has been negotiated and delivered to Lender and is payable in the
State of Illinois, and shall not become effective until accepted by Lender in
the State of Illinois. This Note shall be governed by and construed and enforced
in accordance with, the laws of the State of Illinois, excluding any conflicts
of law rules or principles that would cause the application of the laws of any
other jurisdiction.

          BORROWER:              TELOCITY, INC.
                                 10355 N. De Anza Blvd.
                                 San Jose, CA 95129

                                 Signature:   /s/ KAREN NORTHUP
                                              -------------------------------

                                 Print Name:  Karen Northup
                                              -------------------------------

                                 Title:       Controller
                                              -------------------------------



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TELOCITY, INC.

Prepared by V. Tonga

Loan Amount:        500,000.00 Subordinated Loan & Security Agmt. dtd 5/6/99
                    ==========
Interest Rate           12.500%
                    ==========
Payment              16,767.05
                    ==========

<TABLE>
<CAPTION>
Payment
Number      Date         Principal      Interest        Payment       Balance
------    --------       ---------      --------       ---------     ----------
<S>       <C>            <C>            <C>            <C>           <C>
          09/24/99                                                    500,00.00
   1      11/01/99       10,343.44      6,423.61       16,767.05      489,656.56
   2      12/01/99       11,666.46      5,100.59       16,767.05      477,990.10
   3      01/01/00       11,787.99      4,979.06       16,767.05      466,202.12
   4      02/01/00       11,910.78      4,856.27       16,767.05      454,291.34
   5      03/01/00       12,034.85      4,732.20       16,767.05      442,256.49
   6      04/01/00       12,160.21      4,606.84       16,767.05      430,096.28
   7      05/01/00       12,286.88      4,480.17       16,767.05      417,809.40
   8      06/01/00       12,414.87      4,352.18       16,767.05      405,394.53
   9      07/01/00       12,544.19      4,222.86       16,767.05      392,850.34
  10      08/01/00       12,674.86      4,092.19       16,767.05      380,175.49

  11      09/01/00       12,806.89      3,960.16       16,767.05      367,368.60
  12      10/01/00       12,940.29      3,826.76       16,767.05      354,428.31
  13      11/01/00       13,075.09      3,691.96       16,767.05      341,353.22
  14      12/01/00       13,211.29      3,555.76       16,767.05      328,141.93
  15      01/01/01       13,348.90      3,418.15       16,767.05      314,793.03
  16      02/01/01       13,487.96      3,279.09       16,767.05      301,305.07
  17      03/01/01       13,628.45      3,138.59       16,767.05      287,676.62
  18      04/01/01       13,770.42      2,996.63       16,767.05      273,906.20
  19      05/01/01       13,913.86      2,853.19       16,767.05      259,992.34
  20      06/01/01       14,058.80      2,708.25       16,767.05      245,933.55

  21      07/01/01       14,205.24      2,561.81       16,767.05      231,728.30
  22      08/01/01       14,353.21      2,413.84       16,767.05      217,375.09
  23      09/01/01       14,502.73      2,264.32       16,767.05      202,872.37
  24      10/01/01       14,653.80      2,113.25       16,767.05      188,218.57
  25      11/01/01       14,806.44      1,960.61       16,767.05      173,412.13
  26      12/01/01       14,960.67      1,806.38       16,767.05      158,451.46
  27      01/01/02       15,116.51      1,650.54       16,767.05      143,334.95
  28      02/01/02       15,273.98      1,493.07       16,767.05      128,060.97
  29      03/01/02       15,433.08      1,333.97       16,767.05      112,627.89

  30      04/01/02       15,593.84      1,173.21       16,767.05       97,034.05
  31      05/01/02       15,756.28      1,010.77       16,767.05       81,277.77
  32      06/01/02       15,920.41        846.64       16,767.05       65,357.36
  33      07/01/02       16,086.24        680.81       16,767.05       49,271.12
  34      08/01/02       16,253.81        513.24       16,767.05       33,017.31
  35      09/01/02       16,423.12        343.93       16,767.05       16,594.19
  36      10/01/02       16,594.19        172.86       16,767.05            0.00
</TABLE>

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