There are tools you can run to extract the key. I used to use a thing called Jellybean, though I don’t know offhand whether it supports W7 or not.
“Magic Jellybean” Good software just be careful with the added crap, you have an opportunity to uncheck it pre-install. The trouble with this though is that it’s probably a factory image, and the key won’t be the same as the one on the underside of the laptop, and won’t work with OEM media.