The VideoLAN Forums

I'd like to transcode a video file, and it would be enough for me to change only the container format. Actulally, I'd like to use the avi, but the relevant box is not active. I'm informed that "Depending on previously chosen settings all formats won't be available". Unfortunately, I can't figure out, what those previously chosen settings are.

Ctrl+J when playing the original media will tell you which codecs are used by the video and the audio streams.
And when transcoding using the GUI the Video and Audio tabs will show you which codecs will be used (or not, if copying the streams only) for the destination file.

http://wiki.videolan.org/Transcode