I have three computers in my home office - two desktops and a laptop. I installed Microsoft Window 7 Pro 64 OEM on all three pc's. I used the same installation disc for all three pc's. Each pc had a different motherboard. I was able to activate the OS without any problems of any kind. I've also done four data storage upgrades to my personal pc - hard disk to Kingston SSDNow 200, to Samsung 470, to Samsung 830, to Samsung 840 EVO. Each time I did a fresh clean install using the same installation disc. I did not experience any problems activating the OS. Finally, I've done a couple of "secure erases" and fresh clean installs using the same installation disc. Again, activation was not a problem.
Three months ago I decide to upgrade my personal pc. The upgrade included a new motherboard, cpu, memory, and ssd. I used the same installation disc. Microsoft would not activate the OS. I had to purchase a new installation disc. Looks like I'm good to go for two more motherboard upgrades.
I just read Microsoft's policy for Windows 8 and 8.1. Things have changed quite a bit.