Try downloading Ubuntu www.ubuntu.com
Burn the ISO file to a CD-ROM, place the Ubuntu CD-ROM into your computer and reboot. You can run this without touching your hard drive.
Be aware though that Ubuntu will run about 50 times slower off the CD than it would if it were installed on your hard disk.
Also, Puppy Linux is good to try because it also does not touch your hard disk and it runs from RAM so it runs really fast! www.macpup.org