How do I know which Win 7 key activated which computer?

I've got 3 desktops, and 2 laptops that I've installed Win 7 on. (All legit before anyone asks). I didn't think of it when I installed, but now that I have to reinstall a computer I've run into a problem. I've got my cd keys and disks, but can't remember which key I used on which computer (and don't want to install the same key that's being used in another machine). Is there any way to figure out which key is on which computer? If I go under the control panel it just tells me the Product ID, but not the key. Thanks for any info.