I had a problem with a thread in the XP/Home System process using 100% CPU... triggered by Microsoft's recent DAO critical update
It's a rare but known issue. Should be fixed in SP2. Until then I've reinstalled XP/Home, added SP1 and all the other critical updates apart from DAO, so far so good (but I wish Windows Update would stop trying to get me to install an XP killer...)
More common with a System runaway is a dodgy driver, and it's easy to diagnose what is causing this. If it isn't System it could be legit (running Seti@Home, for example), a bug (Norton Internet Security used to run away on my XP/Pro box and use 100% CPU - killing the process fixed that) or something not so good