You can download an ISO image from many of the Linux source websites (see below), and burn that to a CD.
That is your live-CD that Joe mentioned - just have it in your CD drive when you start up the machine and then press the appropriate buttons on your computer to access the CD drive as the start-up source (it’s usually one of the function buttons - it should display on your screen at start-up before Vista starts to boot). You may need to go into your BIOS around then and enable booting from CD, or at least change the order in which devices are checked and put the CD drive before the HDD.
This will start your system using Linux direct from the CD - nothing is written to the HD at all and you can use it for the testing described above. Most of the links below will talk you through doing it in whatever flavour you choose (Ubuntu is probably easiest - see the link below for a fairly detailed walkthrough with troubleshooting advice too).
Ubuntu - Here
Linux Mint - Here
Debian - Here
By the way you are correct that if you have to try writing zeroes to the drive as per Hamlet’s suggestion it will wipe it of your data. It is designed though to try and make the drive functional again though.
