The problem you are facing may be the result of 1 in a million possibilities, or a combination of a few [can I be any more general than that?]. The good news is that you can use a systematic approach to resolving the issue, but it will take some time.
Some plausible reasons why your -- or anyone else's computer system, for that matter -- isn't working properly may be related (but not limited to) any one or more of the following:
a corrupt file on the hard drive
a corrupt Windows system file
a corrupt or incompatible hardware driver
a newly installed program
a Spyware or virus infection
a hardware error [bad ram, video card, etc]
First, make sure that you don't have Windows XP to automatically reboot during a System Failure. This will allow you to view an error message from within Windows that is associated with your problem. Once you have the error message, write it down and then search for the error message using Google (even if it's a bunch of numbers).
If you don't receive an error message and your computer is still continuously rebooting, I would tend to think it may be a hardware issue -- perhaps one the cards inside your machine isn't plugged in all the way? Try unplugging and re-plugging each card into its respective slot. Make sure the slot and card interface are free of any dust, and then proceed to seat the card into the slot again (ensure there is nothing obstructing the card and never force it to go in).
As for resolving the problems I mentioned above:
For a corrupt or inconsistently reported file on the hard drive: run CHKDSK [check disk] on the drive. Go to My Computer, right-click the C drive, select Properties, and then go to the Tools tab, and select Check Now. Choose 'automatically fix file system errors' and Start. Repeat this process for all hard drive letters in your computer.
For a corrupt Windows system file: There really isn't any way to know if this is the case, unless Windows is reporting a problem with a certain system file. In most cases, CHKDSK [discussed above] should resolve any problems related to your file system that can result in a corrupted file; but if that doesn't fix it, you can try issuing a System Repair. Note, however, that I would not recommend issuing a repair or restore unless you have exhausted all possibilities because you will have to reinstall all Service Packs and hot-fixes once your system has been reverted. As for opting to do a System Restore versus System Repair: a restore will only revert your System Registry (which very well may solve your problem) but does not [as far as I know] restore corrupt system files.