i think cool_spot is right. it is probably the msblast virus that has compromised your system. the msblast behaves in a way that it restarts the pc after some minutes of using it without any errors or whatsoever. this is usally an exploit on the rpc of the system. this is what happens at Windows XP boxes...for Windows 2000 it will give you a DOS attack either at the Internet or to your own system. you could verify if you have been attacked by a blaster viruson a Windows 2000 machine by checking the control panel. if the icons are arranged in a funny way, meaning they are all located in a pane at the left and none on the main window then blaster is in your system
however, we can not also rule out killabee's analysis, since sometimes, but not always, BSODs are caused by hardware incompatibility. since the system may be confused at first when booting but when it settles down after booting up it suddenly recognizes that some of its hardware is messing up the system sio eventually it restarts itself or sometimes it crashes and hangs.
either way, IMHO, try to troubleshoot the problem by first isolating things. first, check out your hardware...are they compatible with each other. veriify if each component of your system is included in its HCLs. then after which looking at your hardware and that all seems to be in proper and working order.. make your analytic mind up to the OS. check if you have patched the system properly and that all critical updates have been loaded...then try to update your antivirus software as well and throw in an anti adware program too just in case :wink:
the bottom line is, when troubleshooting always try to revert back on what you or a user was doing before the mishap. this would give you an idea on how to start on troubleshooting.
hope this helps...