Try rebooting in safe-mode by holding down f8 on boot, it maybe that a program is running and causing trouble with part of the system. If it works in safe mode then reboot normally and click start >run and type in msconfig and select start up and take a look at the programs which start when your computer does, usually its best to play it safe and dont uncheck anything that says SYSTEM in the file path or C:WINDOWS and then uncheck the rest and reboot.