Q : I can watch my video in wmp but when I use it in a script, I have this error :

"AviSource : Could not decompress frame 0"

Why ?

A : You missed the correct CODECS for that movie on your PC.

Being abble to watch the movie in wmp is not enough to be abble to encode it. WMP and Avisynth do not use the same way to read a video. WMP use directshowfilters and avisource needs codecs. That's two different things.

Solution :

Go THERE to find the codecs (video + audio) you need. An an other bad way to do is to use "DirectShowSource" insteeed of "Avisource" in your script.