I think it depends on what version of Win7 you have (retail or OEM). I have a free copy of Windows 7, which I got for attending a Microsoft conference. It doesn't say "OEM" on it, but I have to re-activate when I change just about anything. It used to be that I only had to activate after a motherboard change, but now it seems anything I chnage will trigger it, and I have to call up and talk to someone in India to get my huge activation code - what a pain.
I had to call up and talk to a person after changing the motherboard in my Windows Home Server as well (after one hardware upgrade!).