windows 2000 keep on rebooting

I suspect you have the W32/Lovsan.worm.a or some variant.

Check this site for details:
http://us.mcafee.com/virusInfo/default.asp?id=description&virus_k=100547

Manual Removal Instructions
To remove this virus "by hand", follow these steps:

Apply the MS03-026 patch
Terminate the process msblast.exe
Delete the msblast.exe file from your WINDOWS SYSTEM32 directory (typically c:\windows\system32 or c:\winnt\system32)
Edit the registry
Delete the "windows auto update" value from
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\
Windows\CurrentVersion\Run

W2K has a feature which 'restart when encountered a major systemproblem'. Or in other words: when W2K discovers a bad driver or non-responding service it tries to correct the problem by restarting itself.
You can fix this by starting w2k in safe mode and examine the logfiles.
Once you see which driver/service is giving you the headache reinstall it or disable (depending on if you can miss it of course).
If you know your way inside w2k's SERVICES-manager you can also look into that one and change the setting from 'restart windows at failure' into 'ignore' or whatever fits your needs best.

keep me posted :)