Can I fix an .avi movie to get proper dimentions?

[HartsVideo]

Change the renderer. I believe it uses Overlay by default, which gives you blank screenshots. If you go into View-Options-Playback-Output you can see your current renderer nd choose a new one. If you are using System Default at the moment it is most likely using the overlay renderer.

I wasn't sure which overlay to choose.

[HartsVideo]

When using video overlay there is a trick you can use to get screen caps: play two videos at the same time. Only one player is able to use video overlay at a time. The first player will get video overlay, the second player will end up writing directly to the desktop. You will be able to see the second player's video in screen caps. You can pause the first player if necessary.

This worked. I started with MPCHC and opened another file using VLC, and the VLC screenshot kept the video image.