Converting file types to .avi

Is there a good program available to convert file extensions such as .mkv and .mp4 into .avi files?

The new TV has a usb input so can use it as a media box however a lot of downloads seem to have moved away from the .avi that the TV can process!

I do have a dvi/hdmi to plug the laptop in but want to buy a hardrive and just leave it sitting at the TV as relectunt to let everyone play around with the laptop!

(Own a mac by the way)

http://ffmpegx.com/

Thanks, will give it a try

hey scott i might be able to do it for you at work if you have any more problems, depending on what the original file format is of course.

give me a call if you need any help with it.