Download a copy of Knoppix Linux (
http://www.knoppix.org/) and burn it onto a CD. Its like a operating system contained on a single CD rather than your hard drive. Then, pop this into the computer and boot it up (you may need to press F8 or F12 as you boot up to select the CD drive). If it loads, then you know its not a hardware problem.
If it boots up, then you probably have a problem with your windows installation. Try putting in your Windows installation disk and run the recovery option. If that doesn't work, you will have to reinstall windows.