The way to get this to happen is to install XP but not all the system drivers. Tell use in great detail every driver you installed after XP.
There can be 10 or more (your case should list even a CPU DRIVER!) Here's an example from another machine -> http://forums.cnet.com/5208-7587_102-0.html?forumID=69&threadID=271360&messageID=2699006#2699006
I've tried searching different forums with out any luck.
I have a compaq with xp service pack 2. AMD athlon 64 3200+, 960MB RAM. I just formatted the computer and now the CPU usage for Image name: system , User name: SYSTEM is always at 100% and will not go down. I think this is the reason why my computer is running slow but have no idea. All the other CPU usage threads include svchost and this seems to not be the problem. Any suggestions?