Try following the steps listed on this thread. Chances are that it's a virus, I had the same issue and followed these steps.

http://forums.pcpitstop.com/index.php?showtopic=135158

Also, try downloading the MS Malicious Software Removal Tool from the microsoft website, you might get lucky and have it solve your problem.

Resetting your paging cache might extend the time that the computer is up:

Right click on my computer -> properties -> advanced system settings -> performance -> settings-> advanced -> virtual memory -> change -> no paging file -> set. Restart. Go back to the paging options you were just at and set paging to "system managed size" hit set again then restart.

If the computer won't start at all you might be out of luck. Try switching in another hard drive and see where that gets you.

Best of Luck,
Dys