System Idle Process @ Microsoft.com seems to say that this is normal.

Process Explorer shows some processes (in particular = System Idle Process [using 95% of the CPU]) that System Explorer does not show, but is unable to change the priority. 

Is there an application that will limit the CPU usage of System Idle Process?

Often this item, "System Idle Process", is using almost all of the CPU, making it difficult to do anything (including reboot).  Need to be able to set the priority to "Idle".