Put your windows CD into the CD ROM and hold the shift key (the one with arrow pointing up, just above the Ctrl key on the left of your keyboard). Holding shift for a few seconds will stop the Windows CD from auto running, but the drive will still spin up to read the disk then spin back down again. Once it has spun down you can release the shift key.
Now click Start/All Programs/Accessories and choose Command Prompt. Type in
and hit enter. This will check and replace any damaged system files.