Those days I have been using xvidcap to make nice screencasts, and my xvidcap build generates adding indexes to generated AVI files, so they are not seekable. Fortunately we can (ab)use mencoder to add indexes to them without re-encoding our nice videos. Change to the directory containing your videos and just cook the following shell recipe:

for i in *.avi ; do
   mv "$i" "$i.orig" &&   
  mencoder "$i.orig" -forceidx -oac copy -ovc copy -o "$i" &&   
  rm "$i.orig"
done

If you have a lot of video files you can take your time to enjoy a cup of coffee in the meanwhile. On command completion you will have indexed (and seekable!) video files.