Why do I have poor AVI playback in Windows Media Player (Windows 7)?

ffdshow tryouts is very good so I doubt that has anything to do with it. If you have other codec packs installed I would uninstall them...sometimes that can cause problems. If you have different video editors and converters and stuff, sometimes they add external codecs that duplicate in a way what you already have and you end up with your video stream going through more than one codec/filter.