You say you want to convert AVI to Mpeg? AVI is Mpeg4. Or is it because your PC won't play the AVI file? Could be you need the DIVX codec.
You are wrong. AVI is simply a container format, that is, a box to put an encoded video and audio file in so you can have both in a neat package. Other container formats are OGG and MKV.
An .avi file can be encoded in any number of ways and combinations. divx/xvid for video and mp3 for audio is very common for p2p movies and television.
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Most of the programs I linked run in OSX and are just shell programs that use freeware codecs (e.g. divx) and use frameservers to interconvert from one format to another.
The problem with working with mpeg files (whether audio or video) is that MPEG encoders are typically not free.
Most likely, you want to convert a divx/mp3 (mpeg 2 layer 3) into an mpeg-1 or 2 file. mpeg 2 is the encoding used on DVDs and SVCDs. If this is the case, you will need the divx (ad or payware) or xvid (open source, free) codec, an mpeg1/2 codec, and a transcoder. The third program down on the list I linked (the one with the 8.4 rating) should accomplish this for you.
If you're using system 9 or below, you're pretty much SOL. I'm sure stuff exists but you're gonna have a hard time finding it.
Unless you want to pay for software, you're probably not going to find anything that is monkeysimple to use. As I don't have OSX, i'm not going to be able to just find and give you a program to solve all your problems--you're going to have to do some work. Still, most of these free programs have pretty good automation, but you'll need to read the documentation on the settings. It's also good to read a primer on video formats and encodings, so this won't be such a hassle in the future. The doom9 site has some good information in this regard.
Also, have you tried iMovie? You didn't even bother to tell us what OS you're running, or what model mac you have, so maybe you don't even have it.
