MEncoder is a useful media encoder that comes with MPlayer The Movie Player
To use, you’ll need to download and install MPLAYER as well as the extra plugins. You may also need Real Player, Quicktime, Windows Media Player installed (or an alternative.)
OK I’m always having to lookup the mencoder flags for encoding video files, so heres a useful example:
c:\mplayer\>mencoder.exe input.file -of mpeg -ovc lavc -oac lavc -lavcopts vcodec=mpeg2video:acodec:mp2 -o output.file
of = Output File ovc = Output Video Codec
The rest of the options can be looked up.