Find reason of high CPU usage

Task Manager misleadingly adds the CPU time for Interrupts and DPCs to the System Idle Process. To see the time used by Interrupts use the Resource monitor button and inspect the CPU tab. Alternatively, use a better tool such as Process Explorer.

To get to the root cause you may need to use the Windows Performance Toolkit. Magicandre1981 has a tutorial on using this.

The Windows Performance Toolkit can be obtained from the Windows SDK or the Windows ADK.

More information on using the Windows Peformance Toolkit can be found here, here and here. You may need to download the high quality versions of the videos rather than watching them online in order to see the detail on the screen shots.