You are looking for a Live CD:
A live CD, live DVD, or live disc is a complete bootable computer operating system which runs in the computer's memory, rather than loading from the hard disk drive. It allows users to experience and evaluate an operating system without installing it or making any changes to the existing operating system on the computer. Live CDs are unique because they can run on a computer lacking secondary storage, such as a hard disk drive, or with a corrupted hard disk drive or file system, allowing users to rescue data.
There are tons of ones out there. Here is a list of linux based ones and there is a DOS based one called Doscore.
There is also a list on wikipedia.