As far as I know, .avi containers do not have the ability to store a seperate subtitle track. The subtitles would most likely be hardcoded into the video itself, for example, with fansubbed anime in .avi files.
There are also .mkv files which allow you to have multiple tracks, including softsubs (like you get on DVDs) that allow you to turn them on or off. If this is the case, you'll need a codec pack called Perian http://perian.org/
That being said, I'd recommend MPlayer OSX for mkv files. Read this thread for how to install it right. Haven't tried quicktime just yet (due to lack of full screen. haven't updated to the latest version with that enabled just yet)
I just want the ability to add manually (or the program add them automatically if the movie and sub add the same name) .srt or .txt files to the quick time and also to the front row.