I have seen this on XP and 2000 it usually happens after removing certain malware. Have you done that recently? If so lets see if this works out for you:
Boot using your Windows 2000 boot CD then you will want to Enter the recovery console..
at the command prompt go to
then type in :
If you find, wsaupdater.exe, type
copy userinit.exe wsaupdater.exe
Exit and reboot normally. You should now be able to logon.
In the right pane, you should see
Change it so that it reads:
That should solve the problem, if the malware was the one that caused the issue.
The scary thing is since more malware programs are inserting themselves into the winlogon key, this is going to be a moving target.