On some Windows 8 PCs, there is an issue with high CPU usage while computer is idle.
Load is caused by processes System and “System Interrupts”.

There was no obvious solution for the problem, however there were some clues on the web, to look into network devices.

There are 2 NICs in my PC, one integrated and one, pretty old – in PCI slot. I suspected the problem was caused by the older one.
After multiple tries, I dicovered that after disabling the second network card – CPU load dropped.
However this NIC was in use, for Internet sharing via WiFi router, so I couldn’t disable it

The solution was actually very simple – after disabling one option in device settngs (check screenshot) the load dropped back to normal.
NB! You may need to reboot for changes to take effect!