What is your operating system ?
Is it the same in "safe mode" ?
--> To start in "safe mode", you can see here : http://www.computerhope.com/issues/chsafe.htm
If you can start in safe mode, try to "Restore the system to a previous date"
--> To "Restore the system to a previous date", see here :
Have you the installation CD ?
Regards. Bridget. !