In order to run .mkv you need to install hali media spliter and rename the file changing the extention from .mkv to .avi and you can play it in PowerDVD.
The problem is that comparing with Media Player Classic the PDVD8 use more CPU. I think PDVD8 is not using HW acceleration in that case, but it works.
Regards
Sérgio
Core2Duo E4500 - 2 Gb ram
Geforce 9600GT
Win XP & Sage TV