Check out
http://www.frozentech.com/content/livecd.php for a comprehensive, easy-to-search list of
all available Linux-based Live (=bootable) CDs.
A Live CD you can pop into any PC, which then boots from CD, and chances are good that it'll boot without writing anything to your hard drive.
I personally think that Knoppix is really good. It's an excellent CD to keep handy - the unofficial site for knoppix is here:
http://www.knoppix.net/. Try it and learn how to not miss Microsoft Windows anymore
