Using mencoder to split / trim xvid files

The key is using the ovc copy and oac copy and the ss / endpos options to choose what y ou're cutting.

mencoder .avi -ss 00:01:33 -endpos 00:05:30 -ovc copy -oac copy -o .avi

This will create a .avi based on the original, 5 min 30 seconds long, starting at position 1m 33sec in the original video. Video/Audio quality will be the same as original.