if you really want to do it the legal way:
http://windows.microsoft.com/en-us/windows/get-windows-product-key#get-windows-product-key=windows-7
technically, you can install win7 with almost any key, however it will need to be activated through microsoft's activation process after proper installation.
if you know the key has been used more than once and is still being used on another computer, then it is considered illegal. certain kinds of keys should not be moved, legal-wise.
http://www.zdnet.com/blog/bott/is-it-ok-to-use-oem-windows-on-your-own-pc-dont-ask-microsoft/1561
basically, keys that came with manufacturer-cloned (oem)/assembled (system builder/oem) computers are not technically legal to move around, unless it's a retail key (sketchy, still.)
if you just want to activate your copy with a key, there are enough online sites and ebay sellers that sell keys for cheap.
also, some manufacturers have a preset windows 7 DVD with all their slipstream SATA drivers. on an acer windows 7 laptop, it was 98% impossible to reload windows 7 back onto an AMD chipset without cloning original hard drive or rebuilding a win7 DVD (had several original copies w/SATA support, sp0/sp1/oem/retail/etc. etc.), no matter what though, AMD drivers still gave me sh!t. got so damn fed up... just installed windows 8.1 x64 on it, and though it seemed inferior to windows 7 x64 performance 'cuz of "native driver" support, most of the environment was cleaner and clicky.
if you don't like the win8.1 environment, get used to it and install a classic shell (cleaner interface, personally).
i've noticed on windows 8 laptops, downgrading to windows 7, the performance is degraded, even with SSD's and new intel processors, especially going all the way back to winXP on 3rd gen Intel chipsets.