For problems adding AVI files:
AVI files may use different types of compressions (MPEG4, DivX, XviD, RM, etc.) to encode the video. Hence, you have to have the appropriate codec to re-encode the particular AVI file. Being able to play it doesn't guarantee that you have the appropriate codec for it.
If you have codec packs installed, then this might cause compatibility problems. Codec packs are a compilation of codecs taken from different manufacturers and are installed using improvised installation methods. Thus it sometimes causes problems.
Make sure to install only the official codec for each video compression format. For MPEG-4 v1 to V3, install the codec from Microsoft; for RM, install Real Media, for MOV files, install QuickTime; for DivX, install codec downloaded from www.DivX.com; etc.
You can check what codec compression your AVI file uses by checking it with this simple tool "AVICodec". You can download it from the website: http://avicodec.duby.info