Have you tried Windows XP or Ubuntu Linux on it?
Also, pirated MKV's might not be the way to go to enjoy this htpc (not saying you are a pirate, just an observation.)
It should play all rented or owned discs with the correct software.
Efficient choices for Windows include PowerDVD8 for said discs, or any number of options if only playing DVD and files, such as:
Media Player Classic Home Cinema with DXVA on or Zoom Player with MPC video decoder with DXVA on if just playing content you have encoded yourself with DXVA compatibility.
Best wishes, I would stick with it.....