Mac address of the motherboard.
I've formatted various machines only to reuse the licence and it passes activation fine.
With a new mobo no, you have to do the automated phone call thing.
It's not really the MAC address of the motherboard but a digital signature that ties it to the motherboard's BIOS information.
For versions installed OEM by vendors (IBM, Dell, HP, Sony, etc), they use SLIC which allows them to activate standalone (without phoning home to MS) by linking it with a digital signature of the vendor.
So when you buy a laptop, the 25-alphanumeric product key printed on the COA is not the one that's been used to activate the laptop; they've used the signature version. That's also how they can have a laptop re-imaged and it's pre-activated (because the digitial signature, BIOS information and master product key all tie up).
As others have said, if you buy an OEM licence (normally when you buy a hard drive as that's a loop hole to qualify as a "system builder"), then it's tied to that particular machine which is why automatic activations don't work if you've changed motherboard although phoning MS can sometimes work around the problem.
I bought a retail version of W7 when it first came out and have used that on 3 different systems as I've upgraded over the years. Sometimes, on an upgrade I had to phone to activate, other times, it did it over the internet without complaining.
So with a retail licence, you could "move" your hard drive around, although it's not what the licence is really designed to allow you to do! If it's an OEM licence, then strictly speaking, no, because the licence is tied to the PC the OS is running on.