Okay guys, heads up, as I've recently tracked down exactly such an issue in an 8-core Xeon box that was jammin'. Usual disclaimer about this not being the problem/solution, but here goes.
What's likely happening is that you have a lot of deferred DPC calls. In Engrish, it means, 99% of the time, that there's a bad driver somewhere always begging Windows for attention.
First, while the system is sitting idle, you get LatencyMon
running (don't forget to hit the "play" button). It's going to show you that you have some big spikes. When it does, next thing you do is you check both the Processes and Drivers tab, look anything with a "highest execution time" hundreds or thousands of times bigger than the next one in line.
If it's a process, then you know exactly where to look. If it's a driver, it's not quite as simple, but can be tracked down. Often it's ndis.sys, which means it's one of your network cards (also includes wi-fi cards, USB Ethernet adapters, Firewire, etc), or usbport.sys, which means it's either one of your USB devices, or even your USB controller or hubs.
At this point you want to start disabling the device(s) in question one by one in Device Manager and seeing when the CPU activity goes back to normal idle. After you've found the device, the first thing you do is check the manufacturer's site for a driver update. Also ensure that your motherboard's chipset drivers are up to date, namely with the Intel INF update utility
or AMD's equivalent, as USB, SATA, and sometimes even Ethernet functionality are provided by the chipset itself.