Computer Help! My cpu is pegged at 90+% use at idle ?

I am trying to clean up an old Acer laptop; virus check, defrag, etc..I combined the partitions, optimized, all that. At some point (not sure if it always was) the cpu stays at 95% use all the time. I have tried everything I can find; SVCHOST, disabling winupdates, even rolled back SP3 to SP2 ( I am liking Process Explorer, btw ) even a repair install of Windows (XP). I know this is a fairly common problem, and the pc actually still works, but before I try scraping drivers and formatting this thing (with no guarantee), is there anything you may have encountered to beat this?