Windows 2000 boot problem - This is weird!

[gplracer]

I just built a comptuer for a friend and the wierdest thing happened. After a few days of using the computer the computer could not boot properly. It loads into windows 2000 like everything is fine. I can even here the windows 2000 start up theme. Then all of a sudden there is a blue screen with a small background picture of fans in the middle. I think it is one of the backgrounds that comes with windows 2000. I can hit control-alt-delete and get the task manager but there are on icons and no way to run a program. I fixed the problem by reinstalling the windows 2000 system files. I just want to know what caused this. I thought it might be a virus. Thanks!!!