[CORed]

If you open the case, start with the simple stuff: cable connections, memory modules and boards seated, power connections, etc., and, of course, clean out as much of the dirt, cat hair, etc. from inside. If you're really lucky, this will fix it. I would make the reformat/reinstall the option of last resort. It might fix it, but this could be a hardware problem. If it is, a reformat isn't going to help. If you do manage to get it to boot, Go into the "System" control panel, "advanced" tab "startup and recovery" section, "settings" button, and uncheck "automatically restart". This is probably the dumbest setting in Windows XP, and it is on by default. It prevents you from seeing the blue screen with error message when you have a problem, and just throws you into an endless cycle of rebooting.

Now, of course, the error message on the blue screen will still require some research, as it seems to be Microsoft policy to fire any programmer who writes a comprehensible error message, this policy being most strictly enforced for errors that make you machine unusable, but it at least gives you a place to start.