Do you use Windows Media Player Classic?
Right click when playing a 1080p mkv file, choose Filters --> Fddshow
Find the H.264 codec (at the very top) and change it from fddshow to disable.
Restart the player and play that file again, now it should use Cyberlink PowerDVD Decoder.
I assume you have PowerDVD 7 installed.
Fddshow is a single thread codec, that’s why your CPU only got 1 core working.
The Cyberlink codec is a multi thread codec, so it should use your CPU better and spread the load to all the four cores.