Try running MSCONFIG, and under the General tab select "Diagnostic Startup" then restart your computer and see if your cpu usage has decreased. This will start up the system with only the necessary processes for Windows to run, so if something is running on the CPU in the background and causing the usage to be high when idle, you will see a decrease.
another thing is to go into your power settings "advanced power settings" and see if your minimum cpu usage is at 100%.