Wowweee... That's a tough one.

You got Avid? How about FCP? Maybe that's a tad bit much.

.avi files are notoriously bad in "conversion" softwares. You have a .mov maybe? You could dump it into iMovie and add subtitles from there. It can be a mother of a pain to do.

But hey, I don't know I can help much. Could you explain more?