2 possibilities:
Bad screen -> you need to replace it.
Corrupted OS -> You need to attempt a repair.
Boot using an install CD/DVD that matches your installed OS.
If it's Windows Vista or Windows 7, you'll see an option to "Update". Use it.
If you use Windows XP, get a Linux Live CD and use it to boot.
SAVE all your data to an external drive.
Reinstall Windows XP.
Electro, August 2012
