If you can go with Linux and maybe burn media, Knoppix may be an option. It is free and runs entirely from a read only medium. That way you do not need to bother with potitial trouble form installing a dual boot system and your OS cannot be compromised by internet nasties.
If it does not run on your current machine, you still have your unchanged setup.
Should you really like it, it offers an option to install AFAIK. The installed version is not read only though.