There are a number of things that make the CPU run at 100%. Virus scanning is a favourite, but there is also something(...brain wrack..brain wrack...no I'll have to get back to you) which runs as a result of SQLServer or MSDE being misconfigured on a PC which is also running either Outlook or Outlook Express. Not very helpful, I know, but might help you look in the right area.
I suggest that you try switching off services that you don't need, one by one.
W