Run Belarc Advisor (feerware) to see what apps and possible product IDs are on your machine. You may be able to use cd-keyreader to extract the MS OS product key.
Yes, I'm aware of Belarc, and that works well with XP - Vista, I don't know. But Windows 7 licensing is REALLY complex, compared to earlier versions. And you can't mix an OEM key with non-OEM installation media. And it gets even more complicated when you add additional computers. Once you get to a certain point, you can get volume licensing, but you need at least a minimum number of 2008 servers and/or Windows 7 workstations. Either way, they "check in with home" on a regular basis.