Do you know if you may have downloaded a virus somehow? Do you have the latest updates for Windows? Have you installed a new piece of hardware (i.e. webcam)? How long has this being going on? Try going into safe mode with networking and download the most current drivers if you need them. See if that helps.
Download it, and burn it to CD on another system. Then just boot off the CD, generally you hit F12 to access the Boot Menu. If the issue is still present while running off the CD, then it's hardware. If it doesn't shut down after a few minutes, then it's software. The good news is you can use the Ubuntu CD as a way to backup your files.