You can also use VLC player to take screenshots. Open an avi (or any other playable file) using VLC player

- then press the right mouse button
- now select 'Snapshot' from the list
- the image will be saved in .png format (by default) in your My documents> My Pictures folder. The good thing about png image is that unlike .jpg, it is exactly the image you'd see while waching the docu (or anything else). Specifically, it's more helpful if you're looking for a top quality encode from std DVB or HD DVB, etc :)