An oldie, but a goodie, and there is always someone out there who doesn’t know about this. Often, one encounters video files that do not have an index or have a broken index- this is almost certainly the case if your media player or PVR refuses to allow you to fast forward within the file. Easily fixed under Linux using the following mencoder incantation:

mencoder -idx input.avi -ovc copy -oac copy -o output.avi

As a bonus, this apparently also repairs broken interleaving.

Some more great mplayer/mencoder hints, including tricks such as appending multiple AVI files are here.