Hi,
As you may find in many guides here.
I assume that from the file extensions you're talking about *.idx/*.sub you used vobsub to extract the subs from your movie.
You need also VSFilter.dll in your Avisynth's plugins folder.
Besides that you'll need a line in your script like:
Code:
Vobsub("c:\path\subtitles.sub")
But if you're saying that playing the script through wmp you can already watch it with subs, it's because you've followed these steps and you'll be alright.
Cheers
PS-When you forget to include something in a post, try the edit button on it instead of reposting