Have you tried opening up your case and taking a look at your heatsink? Sometimes dust and garbage gets into your fans and heatsink and chokes out the air flow so your processor begins to overheat. If that is the problem use a can of compressed air to clean out the inside of your system.
But you may also have corrupted boot files.
Good advice by Bullzi.
Not all XP setups will give this option though. I would try his suggestion first, but if you don't see this option try loading up into safe mode to see if it can even do this. If it can, the problem is fixable. If it can't, it's likely a hard drive issue.