I just built a computer So i have No operating system on it at all. I Used an old version of Windows XP Professional That I began to Install, but Now i do not have the CD Key. How Can I Uninstall Windows So I can start With A Clean Slate?

You need a CD-key only? There are some CD-key generator, though not legal, but you can generate some CD-keys by it & can use.
It'll be best if you can mange a fresh Win-XP service pack-2 CD from your friends or from somewhere else..

Dont go mucking around with pirated software, especially windows software. If you are going to install a legal windows on your hard drive it will ask you if you want to format your hard drive before installing. By selecting "Y" you will have a clean install. If you want to install another OS you will need to boot up your PC in DOS and format your hard drive from there.