Ashlee, You can get the key by using Belarc advisor which can be found here:http://www.belarc.com/free_download.html
The key will be under Software Licenses. My suggestion is that you copy the key, paste it to Notepad, and print it out.
You can use a borrowed XP disk and your key-just make sure you use the correct version.
If the installed version is Home, you must use a Home disk. And if Pro, obviously you would need a Pro disk.
There are other programs which will do the same thing as Belarc-others can supply the names of those programs.