After booting my Win2k box just now I couldn't log on because my profile was corrupt. Only Windows knows why - everything was shut down ok, but hey, it's Windows :-P
Here's what I did:
Go into Control Panel - Users
Change the name of the corrupt profile (originally called Vincent) to something like VincentCorrupt.
Make a new profile with the old name (Vincent).
Log off, then log in as the newly created account (Vincent).
Log out and you should be able to log in as the old one (VincentCorrupt).
If this works, go back into Control Panel - Users and delete the new one (Vincent). Now rename the original one (VincentCorrupt) to it's old name (Vincent).
You should still be able to log in as normal now.
It's a bit weird, but it has just worked for me - honest :-)
Or you could just give up and use another OS ;-)