|Note that XP does restart once or twice during install, but if it just shuts down, especially if it does not pick up the install at the same point, you may have a hardware issue. Test your memory first with a bootable CD you prepare with Memtest on it by booting to it. This is also good to identify hardware issues, since it operates outside of Windows so any problems cannot come from Windows. Run Memtest through all tests, no errors are acceptable. If errors or if it freezes or restarts, retest with one stick of memory at a time to confirm if the problem is with the memory and which one. Replace as needed. Once you can confirm that your memory runs without errors, test your hard drive with a bootable CD created with the DOS version of your hard drive mfg's test utility, run the longest test available. Testing with the utility from your hard drive mfg. is best, but Seatools from Seagate will work on nearly all conventional hard drives. The hard drive utility will repair minor errors it finds, but many small errors or something major will mean replacement of the drive is indicated.|
Please note that neither of these will in any way effect the CPU usage, which was your initial question. This is noted so anyone reading this will not confuse these tests as having anything to do with your initial issue/question, but the
shutting down you experienced during Windows installation and possibly other stability issues with Windows.
Without knowing what plug ins you are using, we cannot tell this, but it is possible that even one plug in can do this, if it is poorly created or is designed to cause problems (like malware is). To test this, just disable all plug ins and then reactivate just the ones you use once you see the results of the test. Use add/remove programs to permanently remove anything you do not want/need. I recommend the minimum amount of plug ins for comfortable browsing, even for more powerful multicore systems.
Again, if it does not test out that it was caused by a plug in, uninstall nearly everything that you do not use and run Malwarebytes in Safe Mode because you most likely have something there.
If this does not prove the case, then i must suspect that when you reinstalled Windows XP, you did not begin by deleting all partitions on the hard drive and creating new partition(s) before installing Windows, which might indicate that you had a Root Virus that survived the reinstallation (hence the name 'root') in some small way and is still there, slowly working itself deeper through your system again. If this is the case, the best solution is to delete all partitions and start over. ((Install Disk>Custom>Partition/Format....))
You have to be a little bit crazy to keep you from going insane.