Just a thought, Svchost is something you're stuck with on xp, it can't run without it. Sooooo, if it's hogging the cpu at start up, I think an effective strategy would be to consider it from the device viewpoint.
Since it queries what services need starting in respect of soft & hardware & you've turned off all the services you think you can get away with, I would if it were mine, disable devices in turn one by one until I came across the offending piece.
Your soundcard got me thinking about this. In my experience, the biggest problem with XP is device drivers (media center is worse, far worse, god only knows how much of a pain vista will be), I reckon you might chance across a dodgy device sooner or later. Tip: don't kill your monitor or mouse
