This was recently posted on techrepublic. I haven't tried it myself:
USE CMD.EXE TO RECOVER FROM A FORGOTTEN PASSWORD
Forgetting the administrator password may seem like a catastrophe, but
it doesn't have to be. We'll outline several possible solutions to the
problem. If these don't work, however, you'll need to reinstall the
system.
The first resolution applies to computers that have another account
that is a member of the Administrators group. If this is the case, log
on with that account and change the password for the administrator
account.
Another method that relies on Windows 2000's logon behavior allows you
to bypass logon and change the administrator account password. Here are
more details about utilizing this method.
When Windows 2000 boots, it displays the logon prompt for a while and
then (if no keyboard or mouse activity occurs) starts the LOGON.SCR
screen saver. If you can access the system's files, you can replace
LOGON.SCR with CMD.EXE.
When the logon timeout occurs, Windows 2000 launches (in the context of
the system account) the command console rather than the screen saver.
Use NET USER to change the administrator password or launch the
Computer Management console (COMPMGMT.MSC) to accomplish the task. If
you choose the former, issue the command NET USER ADMINISTRATOR
<password> where <password> is the new password you want to assign to
the administrator account.
How you replace the LOGON.SCR file with CMD.EXE depends on whether the
system uses FAT or NTFS. With FAT, you can boot from a Windows 9x or a
DOS boot disk. But for NTFS systems, you might need a solution such as
Winternals' NTFSDOS to access the file system or access the system
across the LAN. Regardless, once you have access to the
%systemroot%\System32 folder, rename LOGON.SCR to LOGON.BAK and then
copy CMD.EXE to LOGON.SCR.