Do you have Norton on that computer? Every computer I had with Norton had a CPU usage problem, my solution was complete format and use another firewall as in my personal experience with Norton; It sucked bad.
I'd recommend Zone Alarm if you wish to purchase one, and if you need an Anti Virus program, go for AVG, they have a free edition and I've never had a virus I couldn't take care off with it
http://free.grisoft.com/doc/1
Hope you get it sorted mate!
--Lee