Since you seem to be so helpless that you can't even make a simple Google search:
1. Open MPC
2. Open options
3. Go to Output
4. Select Haali Video Renderer
5. Click OK
6. Open a video
7. Go to File -> Save Image
8. Save image where you want your snapshots to go, make sure to save in JPG
After this, press F5 whenever you want to take a screenshot and it automatically saves a JPG snapshot to whatever folder you saved the first image in.
If you experience slowdown with Haali Video Renderer, VMR7 (windowed) is the fastest renderer which allows you to take screenshots.