What parts exactly make a computer? Is it just the hard disk drive because this is where the program is stored
That's defined in the EULA as:
1.1 Installation and use. You may install, use, access, display and run one copy of the Software on a single computer, such as a workstation, terminal or other device ("Workstation Computer"). The Software may not be used by more than one processor at any one time on any single Workstation Computer.
Note that the last portion "may not be used by more then one processor" seems to preclude use on multiple processor machines. It does- that is actually from the XP Home Edition EULA. The Pro version is more verbose but allows for a lot more:
Installation and use. You may install, use, access, display and run one copy of the Product on a single computer, such as a workstation, terminal or other device (“Workstation Computer”). The Product may not be used by more than two (2) processors at any one time on any single Workstation Computer. You may permit a maximum of ten (10) computers or other electronic devices (each a “Device”) to connect to the Workstation Computer to utilize the services of the Product solely for File and Print services, Internet Information Services, and remote access (including connection sharing and telephony services). The ten connection maximum includes any indirect connections made through “multiplexing” or other software or hardware which pools or aggregates connections. Except as otherwise permitted by the NetMeeting, Remote Assistance, and Remote Desktop features described below, you may not use the Product to permit any Device to use, access, display or run other executable software residing on the Workstation Computer, nor may you permit any Device to use, access, display, or run the Product or Product’s user interface, unless the Device has a separate license for the Product.
This allows for use with 2 processors. Then again, Dual processors (note that dual processors are not the same as a multi-core, whereby two cores reside on the same processor) are not usually seen in consumer oriented designs. In fact, very few consumer motherboards even support multiple CPUs.
What happens when someone has to replace their HDD? Do you have to get a new registration number for windows?
No. the Internet Activation might succeed anyways, and if it doesn't you simply perform the phone activation. No idea what a "registration number" is, but if you mean product key then maybe, but if that's required they will give it to you through phone activation.
Long story to the original question, The license says you can only install it on a single computer <at one time> if you install windows to Computer A, you cannot then install it on Computer B (well, it might be <possible> but it's not covered under the License agreement (which, despite peoples claims to the contrary, they agreed to when they checked the box)). You can remove the windows install from the first computer and install it on the second, or you could even have some freaky weird setup whereby only one computer can ever be using it at the same time. Any barriers to people exercising these particular abilities are completely fabricated. Some people simply will not perform the phone activation, purely out of stubbornness, and then say that MS is "preventing" them from activating on a new machine, or when they replaced their HDD (which, I might add, will activate fine by the internet on the same hardware anyways).
