I knew that if you got an OEM edition it would bark at pretty much any change. But I'm a bit peeved that you can't even tinker with your core components on retail without needing a phone reactivation. I just think it's a bit on the extreme side of anti-piracy tactics.
Generally, you can tinker with quite a bit with a Retail edition. But in Vista (and I assume Win7), simply changing the disk will trigger re-Activation. MS has never identified how their latest "point system" works. They haven't commented on the scoring system since 2002 or so.
Note that a failure to Activate on Win7 won't necessarily stop you from using the computer. Starting with Windows Vista SP1, Microsoft removed the "reduced functionality" mode. Delayed Activation, based upon what I've seen with Server 2008, generally results in nothing worse than a black desktop background, pop-up warnings on the desktop, and the inability to do automatic Windows Updates of anything but critical updates.
Some people have experienced worse effects (a completely black desktop), but I don't believe those were intended by Microsoft.