I'd go with Ubuntu
http://www.ubuntu.com/getubuntu/download

Download the CD image, and burn it to a blank CD

You can try it out without changing anything by running it off of the CD
(It'll be a tad slow, but you can have a look around and see what it's all about without actually installing it)

Re: Yahoo messenger
Use Pidgin, it's a multi-protocol IM client
comes pre-installed with Ubuntu