Sorry I missed 4wd`s post.-kartal
You are forgiven my son
Actually, this wouldn't be all that hard to do using AutoIt, eg:
1) Extract archives to two temp directories using UniExtract.
2) Compare and report the contents using any diff program, (BeyondCompare, WinDiff, etc).
The updating of one archive based on the result is a little more interesting but should be possible providing the comparison program can output a plain text difference list, then just pass it to the appropriate archiver to update the chosen archive.
Simple!
Let me know when you've got it done
EDIT: A little poking around on SourceForge reveals
CompareArchives. Will do ZIP, EAR, WAR and JAR. It's written in Java so it's platform independent and if you're able you could modify to do other types. Output is in the form of a HTML report, (hardcoded to use IEXPLORE.EXE on Windows).
Another EDIT: Thinking about it, the AutoIt, (or similar), way of doing it is probably the only way you're really going to get something like this done w.r.t RAR archives. Other programs are able to do ZIP, (and others), because they are open-spec algorithms - RAR isn't. You're always have to have WinRAR installed if you want to do more than just see the results, whereas other programs can actually update archives.