Windows 8.1 System process using High CPU

Thanks to tumchaaditya for suggesting sfc /scannow. It identified a corrupted printer driver and associated .inf file: CNBJ2530.DPB and prncacla.inf.

That led me to the solution: http://www.sysnative.com/forums/windows-update/7702-guide-cnbj2530-dpb-and-prncacla-inf-corruptions-windows-8-8-1-a.html

This page contains detailed instructions on how to fix what is apparently becoming a rampant problem with windows 8/8.1. Running "DISM.exe /Online /Cleanup-image /Restorehealth" replaced these bad files with new versions downloaded from MS Servers. Rerunning "sfc /scannow" confirmed everything was healthy again.

Following a reboot, the NT kernel is now using 0.13% of CPU instead of 13%.

In my case it was a different problem, but same symptoms. It was the RunFullMemoryDiagnostic task that somehow never ended. Just disable it in the Task Scheduler, inside Microsoft > Windows > MemoryDiagnostic > RunFullMemoryDiagnostic.