Fouled-up power management?
Still a hotfix, still a "contact MS" hotfix, I believe, but available directly from several makers.
http://support.microsoft.com/kb/896256/en-us - this one? - yes, still a "contact MS", when it is absolutely certain that Windows XP will NOT correctly calculate loading across a dual core or HT CPU - with the result that a 100% load appears as 50:50 over the two cores / logical units and does not promote the power state beyond minimum.
About time MS did SP3, or at least put some of these "known bug" solutions on general release.