Try starting your computer in Safe Mode and performing a system restore.
To do so:
Restart your computer and immediately start tapping the F8 key on the top of your keyboard. Tap..tap..tap, do not hold it down. Continue this until the Windows Start Menu appears. Then use your arrows keys to select the option called Safe Mode and press Enter key.
Once your Windows Desktop appears, click on:
Start Button->All Programs->Accessories->System Tools->System Restore and select a restore point dated prior to your problem. If successful, this should solve your problem.
Restart your computer the normal way and see what happens.
Another reason for this type of issue may be a newly installed device. If the above did not solve your issue, try disconnecting or removing any new devices (ie: printers, webcams etc) that you might have installed