cpu usage way too high w/ nothing running

[runLoganrun]

my cpu is periodically running at 50% + for extended periods of time, with nothing running, and then drops back down to 0 for no reason that I can tell. I've done a virus scan w/ Avast home edition, and gone through my startup items, but can't find a reason why. it's a pretty new install of windows (vista x64), about a month old. does anyone know what could be causing this?

thanks,
rlr