Shoutbox

Ok, I'm assuming you are running XP.

Step 1.
When your computer is starting up you have to go into the BIOS (there will be some key/s you have to press to get in at a particular stage before windows starts loading).
Step 2.
Change the "Boot Order" so that your CD/DVD drive is the first one to boot from.
Step 3.
Save changes in bios and then let windows load.
Step 4.
Insert Windows XP CD and restart your computer.
Step 5.
Follow the prompts to format your computer (I'm unsure on exactly what you have to do from here, but I'm sure you should be able to tell)

Before doing this please make sure you have any important information off of your computer.

If you need more information then I'm sure there will be someone who can help (though I don't think you should)