Problem with some software installed or some change made to the system.You need to open the computer with safety mode, uninstall the software or change.
If not open with safety mode-- problem may be hardware or boot level files affected.
To open safety mode, during booting press f6 or f7.
Tip:
- It's called "Safe mode", (not "safety mode"):
- Press F8, (not "f6 or f7"):
http://support.microsoft.com/kb/315222