Eastern is right that it could be any number of things, but it could also be quite simple to fix.
The first things I would try would be a System Restore: START>ALL PROGRAMS>ACCESSORIES>SYSTEM TOOLS to a date before the problem occurred.
If still no joy, I'd try My Computer, Right-Click "C" then Properties>Tools>Check Now (check for errors) and tick both boxes. You may need to reboot to activate the check.
There are lots of other things that may help, but those would be my first tries.