Here is a nice full featured email client that runs on its own, you can just copy it to a USB stick.
(Scroll down and download the "Floppy Office" set of programs. When there, grab the zip folder that says something like "npopwin32" and unzip the program and the readme to your usb stick.)
just put a link and a write-up to this in the Internet section. Thanks!
nPOP's official website is: http://www.nakka.com/soft/npop
Personally, I'm a complete fan of Portable Thunderbird and don't plan on switching over anytime soon. It along with FreePOPs are quite the killer combo.
Rob Loach [Website] [Projects]
I like PortableTB too, but once again, just looking out for the people with small drives.