Did you have a virus by any chance? It may be that the registry has become corrupt.
You could try a System Restore in safe mode.
Restart your computer. When the computer starts you will see your computer's hardware being listed. When you see this information start to gently tap theF8 key repeatedly until you are presented with theWindows Vista Advanced Boot Options. Select the Safe Mode option using the arrow keys. Then press the enter key on your keyboard to boot into Vista Safe Mode. When Windows starts you will be at a typical logon screen. Logon to your computer and Vista will enter Safe mode.
You then need to return the system to an earlier time
This article takes you through the steps.