Windows 7 Can Use 5 / 5 Digit Code From Old Computer?

I currently have a friend who is in need of a new computer, which I can provide for him, however it has Linux on it, which he will not be able to use..while I interned at a computer repair shop a few years back, every time a hard-drive needed re-imaging, and the customer had no idea if they even had the 7 disk (which we know that most users buy them already installed) The owner of the shop would use a generic?/burned copy of windows, and when the license 5/5 digit code was requested, we input the license info off the MS windows sticker on the side..it always worked, at least there. My question is, how do I get Windows 7, do i download it off the net? He had every disk, from xp to 7, burned so we could use it for install..is this possible in the real world? My friend cannot afford to purchase a new license.