It is difficult to resolve those problems if you do not know when they started , after installing new sofware or hardware .
Then you try to uninstall the installed software , or take the pci card out you have installed last .
Another method is :
Take all pci cards out , keep only your video card and restart
If you can startup win2k , put the cards one after another back and restart every time untill you get your error again .
If you cannot startup from the beginning then it could have something to do with the video card but also with bad contacts of the agp (pci) or memory slots .
How about the power supply in your computer 250w or 300w ?