"and in the end we had to turn it off"
Why?
Have you by any chance left a Windows disk in the Optical drive?
You haven't said which version of Windows you are running. Try restarting the machine and repeatedly tapping F8. You should get a menu of startup options. Try "last known good configuration". If that's not available, choose safe mode. Sometimes, simply starting in safe mode, and then restarting in normal mode is enough to fix the problem.
It's possible that there's a physical problem with your disk drive - how old is the machine?