You are able to use a LiveCD/USB. This allows you to boot the computer from your CD or USB, and be able to run the full-fledged OS from memory. If you use USB, you're able to add persistence as well, which allows you to save files without them being deleted upon shutdown.
Ubuntu's standard install CD is also a Live CD. Google, as well as PenDriveLinux have fantastic guides on how to set up a live CD/USB.