Hitsu, the best way to find a specific file converter from A file to B file is to just search up on Google. Please don't take it wrong but asking a general questions will return a general answer
As for a specific converter that converts from RMVB to AVI I do not know. (notice specific-ness there? ^^) In your case what I would do is go to Google and search [I use quotes too on my searches] "How to convert from RMVB to AVI" or something along those lines....if you get no hits with the quotes " " then take out the quotes and ask the same question. It's how I figured out how to convert from OGM to AVI
