Have you got Windows OneCare or (more likely) Windows Defender on your PC?
It's possible that MsMpEng.exe is your problem (because it's found in those applications). Wait until things grind almost to a halt and take another look at its CPU usage. You might well find that its up in the 90s. (If so, the answer would be to uninstall Windows Defender and to install alternative security software).
Alternatively you might find that Svchost.exe is showing a really high figure. For that you'd need to follow the instructions here:
http://www.technibble...-100-cpu-memory-leak/
Chris