I'm having the same problem with my wife's Windows 2000. It will boot up to the password screen and it will let me in when I type the correct password, but within 15 seconds tells me it is saving settings and logging back out - then I'm back to the login screen.
Here's the big problem - We don't have the Windows 2000 CD anymore, or if we do, I'm not sure if we have the login code anymore. I don't have a boot disk either. Before it crashed I'd updated the virus-scan so all I need is to log in and run it, but I can't login. Anybody have any ideas?