Let me be the first to recant my previous post.
There is a program out there that does this exact voodoo magic that you want, I just remembered about it because I am using it during a mailserver transition to get all of the users passwords so that I can put them in to the new server.
It's called dsniff. It's typically a linux program, but there appears to be a fairly active windows port of it. Search on google for the directions to run it, it looks like you might need to install some special drivers. All I needed to do was "apt-get install dsniff", why are things so much harder on Windows?
