The problem is that CoreAVC is not compatible with most fansubs (using hardware acceleration, CUDA) due to the nature of how they encoded and they need to be compatible in order to use it and it's not free. It should work fine if you don't use hardware acceleration.
If you have a compatible graphics card like nVidia Geforce 8xxx, 9xxx series or higher or ATI Radeon HD series, you can probably take advantage of DXVA which is supported under Media Player Classic and should be compatible with all the fansubs out there. I suggest trying the instructions at the MPC HC website:
http://mpc-hc.sourceforge.net/DXVASupport.html