First, you know that doing this will wipe out all of your files, internet favorites, pictures of your best friend barfing all over the teacher, etc...?
Call Dell, they may supply you with one for a small (or not so small, I don't really know) fee. Otherwise you need a valid Windows CD with a key, if you can find an OEM copy you should be able to use the Windows key off your computer, but only if the Windows CD you use is an OEM version.
If you go the non-Dell CD route, you will also need to download all of the drivers for your system off Dell's website, probably using a friend's computer.