Hiya henrys
I'm not sure what OS you're using but if you're using Win98 I'd use ScanDisk, if using 2000 or XP go to My Computer, and d/click your harddrive(s). This done r/h/click and select Properties. While you're there you can clean your drives as well if you like. Getting back to your problem, click the Tools tab, Check Now and place a tick in each of the boxes. When you retreat to My Computer they'll say you'll have to reboot. It may well take a while if they really need it.
Hope this helps.