If your computer has an integrated GPU (Intel HD) and a NVIDIA graphic processor, then Media Player Classic will uses integrated GPU by default and you can't change it. But luckily, there is a solution:
- Download MPC-HC 32bit version and install it.
- Download CoreAVC v3.0.1 and install it.
- Open MPC-HC, go to View \ Options \ External Filters, click "Add Filter..." button and select CoreAVC Video Decoder. Make sure the "Prefer" is checked.
Now play your video and you can see the MPC-HC icon in the NVIDIA GPU Activity, there is also a CoreAVC icon in the Taskbar :)