Windows 2000 Reboots after logging in

When I start up a system running Windows 2000, it gets all the way to the login. It starts to log in, the screen switches to black, as if it were going to adjust to my user settings for screen resolution and all that, but when the display comes back, it is rebooting. I have tried safemode, and safemode with dos prompt. Same result. What can I do to get windows to log in?