http://www.magicaljellybean.com/keyfinder.shtml

Run that on her computer to find her key.

Whether you can use your cd with her key depends on where it came from. Her key will be an oem key. If your cd came with an hp or dell or something, than you can probably use it. If your cd is corporate or if you bought it at a store (retail) then you can't use it. If you try a key that doesn't match the cd, it will call it invalid and not accept it. Of course it's possible to change a retail cd to an oem cd and whatnot, but that's another story.