Does the computer reset itself after you try and start it? Any errors or blue screens flash up just before it resets? If none of the options work, you could boot the XP CD and try a repair installation. If this doesn't work then it's best to reformat it and put Windows back on from scratch. If you do this, you'll need to take out the hard drive and put it in another computer first so that you can copy any data you need to keep before you reformat.