How do i find product key for computer. i bought it at pawnshop so i have no package?

my comp says my windows 7 is expired and not genuine and to activate,but my product key on the label is from microsft. i have a laptop i got a week ago from pawnshop. when i log into windows vista (other operation), a start up repair comes up, but unable to find a problem. i NEED help!!!!!! without buying another product key for windows.