You mention there's a problem as "administrator".. Can you log in as someone else? Can you boot the computer into Safe Mode?

How To Start In 'Safe Mode'

If it will boot in Safe Mode, can you scan it with your antivirus and antispyware programs?

My first thought is a power supply problem.. Have you tried replacing the power supply?

If you have a copy of Service Pack 4 for your computer, have you tried installing it while in Safe Mode? (I'm not sure whether it can be done or not but it should fix any corrupted files if that's the cause.) See the SP4 download at the link below. It can be downloaded on another computer and burned to a CD for installation.

http://www.microsoft.com/windows2000/downloads/servicepacks/sp4/default.mspx

There are lots of possible reasons for your inability to start the computer up and some of the fixes involve using the Windows 2000 CD.. Do you have it?

Hope this helps and let us know how it goes.

Grif