I just downloaded the win7 home premium 64 bit ISO off digital river and I'll soon transfer that to a usb flash drive and will use it on a new pc build.
Will the ISO require me to enter a product key? I've read there's a trial period...
Will my product key on my laptop/old premade desktop (with same win7 version) work...?
Depends on what version of it you have, some dont ask at all and some will ask but you can skip it and enter it later. If in doubt just install it in a VM and you will have the answers you seek.
I'm also planning on a pc build for a friend and we have an old Gateway with vista installed, but then i got a free upgrade to win7 free from gateway a while back, will that product key work as well?
It will work because I used a digital river .iso for my build. The only catch is that it has to be the exact same as version of win7. My laptop luckily has 64-bit version of home premium. If yours has a x86 version and you want a 64-bit version then you're out of luck. Unless you stumble across one of those magical free ones.