I know more about this stuff for WinXP than Vista, but I have to assume it's all pretty similar.

OEM copies grunge around in the BIOS ROM looking for evidence that they're seeing a board from the same manufacturer. So, if you do a HD transplant across two machines from the same vendor, you've got a good chance that it just works.

Otherwise (and perhaps regardless), all of the other Windows Activation logic kicks in, where they try to diff the "current" machine against the "activated" machine. Assuming they're "within tolerance", then re-activation shouldn't be required. If they're completely different, then they tell you that you need to make a phone call.

At that point, it's not really a big deal. You say that you did a brain transplant and that you're retiring the old machine or wahtever, and they give you the magic activation code.