There should still be a key sticker on the laptop if it was bought from the factory with Win 7. Look under the battery for the sticker.
If someone installed Windows 7 and it’s a legal copy, use something like Magical Jelly-Bean Keyfinder to save the key for future use.
Keyfinder sometimes doesn’t work on factory installs due to them using some sort of generic key, but if it was installed by an end-user, it should work.
You might have to disable any security software to run Keyfinder, but it’s legit, not a virus.
M$ took down the downloads of the actual install DVDs last year, but they were fortunately mirrored and are available to Torrent. Untouched and perfectly legal to download if you’re licensed to use the software.