AviSynth is a frame-serving program that provides a way for you to take a previously compressed video, decode it into raw images, and then manipulate those images prior to recompressing them into a new video. You can also use AviSynth to add effects and other types of processing to your videos on-the-fly during playback, assuming you have a sufficiently powerful computer capable of applying the effects in realtime. It's highly useful software for video transcoding (e.g. ripping and recompressing DVDs or Blu-rays), but whatever SVP is trying to do apparently causes problems in this instance. It isn't AviSynth's fault, though, so if you uninstall it, SVP will probably just complain that it can't find AviSynth.