Do you have anything worth saving on that machine?
If yes, get hold of a USB drive and a linux live CD. Boot from the CD, copy all essential data onto the USB disk.
In either case reinstall Windows. It's in all likelyhood the fastest, easiest and safest solution to your problem.
P.S. If you computer fails and reboots while booting or running from the CD it's a hardware problem. Contact your computers manufacturer for support and warranty issues.
Oh, and before you dismiss this advice: I have a bachelor in Information Technology and have been working as an onsite support technician for the last 3 years.