Here's a summary of reactivation behaviour:
Small hardware changes will not be flagged as a change of PC, so simply replacing the HD will allow an internet activation like before. What exactly constitutes "small changes" Microsoft doesn't like to exactly say, although they do have a FAQ on it, with guidelines.
However, if Windows thinks it's a different PC with the same key, then you see the following behaviour:
Retail boxed copy from a shop: call the Microsoft activation helpline (the activation wizzard will guide you to it). You will be asked a couple of questions, either by a person or by pressing option buttons on your phone. Answer them the right way and you will get an activation code. And yes, they do take your word for it.
TechNet download copy: you get 10 activations with the same key on different hardware. Then it's the telephone. Personally, I've not activated it on so many different PCs that I've needed the phone.
After 120 days, the activation status resets and you can internet activate on another PC without having to do the phone activation.
Note that application activations such as Office, will have variations on these rules. However, all TechNet products get 10 activations and some server products have a Multiple Activation Key.
Just a quick note on a TechNet sub: if you've never subbed before and you want to buy a copy of Windows, Office or other MS product, sub to TechNet instead, it's phenominal value for money!
For a moderate amount of money, you will get masses of software that you can use forever, every version of that software and 5 keys for each version and each key can be activated on 10 different computers. You also get speedier support from TechNet pro and a helpline you can call.
Note that it's not supposed to be used in a commercial production environment. So, running this forum on Windows Server say, would not be allowed.
I use Windows Server 2008 R2 Datacentre Edition at home. Just this on its own is worth around £6000, so it gives you an idea of how much value TechNet gives you.
