I'm trying to guess if what you mean is that you are experiencing the Windows Blue Screen of Death.
Alternatively you are encountering the "To start, Press Ctrl Alt Del" window. If so, do that !
If it's the Blue Screen, turn off the computer. Start it again and before Windows usually starts press the F8 key -- Windows may start in Safe Mode. You won't have all the facilities but some. Hopefully when you close Windows, later it start again normally.
Otherwise you need to start the computer from the Windows CD and run the repair installation routine.
If none of this helps, you need to see a repair shop.