Similar issue I had too, and ended up boiling it down to the Microsoft Audio Endpoint Builder service. Found if I went into device manager and disabled the ATI HDMI Audio driver (Don't even have it hooked up) viola! Problem solved.
Thank you ZerouEx! This was what solved my 100% CPU usage problem too. I went into device manager, and under 'System Devices' there was a listing for "High Definition Audio Controller". The very instant I disabled this entry, my CPU went from 100% down to the types of levels (<5%) that I would normally expect from a mostly idle machine.
Thanks to all for your suggestions, and specifically to ZerouEx for the one that worked.