Boot into safe mode if possible. If so, it's quite possibly a driver issue (did you do any software updates before the problem?).
In safemode, open a command prompt box (DOS), and goto C:\Windows then type chkdsk /r
If that lot doesn't work, perhaps a boot up with the original windows disk and on boot, perform a repair (R).
I didn't see which specific operating system btw. Guessing at XP.