What causes a computer to reboot continuously?

You have a problem with the OS. Try booting in Safe mode. If you don't know how to do that on your machine, try tapping the F10 or F12 buttons at startup. Possibly F8, too. Depends on the BiOS. If you have a recovery disk shove that in the drive. Also try unplugging it for a few minutes. If none of this works, switch to Linux.