FRUSTIX is a Linux OS which is distributed in an ISO image.
You can burn it to a CD (443 MB) and place it in your CD drive. Booting from this CD gives you a complete basic Linux with some apps and some games.
You don't need to worry about a harddisk, because it runs fine without one and doesn't touch it, if there is one.
If you need to save new created data, you can eject the FRUSTIX CD and use UDF formatted CD-RWs.
So if you wanna test it, then download it!
After resetting your computer and / or taking off the CD, you can run your previous operating system from HD again (if there already was any, before?).