Windows 2000 will not shutdown or logoff

I have recently installed Windows 2000 Professional on a new PIII – 1GHz pc in an NT domain environment. It worked fine for about 3 weeks, but now whenever I try to logoff or shutdown, the hard drive light flashes for an instant and then nothing else happens. I can continue working, lock the pc, etc. I just cannot quit. If I lock the pc and then unlock with a different admin account, I can shutdown from the “logon” screen. Any ideas?