"Certainly a scam. Microsoft does not as a rule scan your computer for hardware changes without asking, so they wouldn't know what's in your comp."
totally wrong. The authentication system checks for changes in your hardware, awarding different point levels for different changes. If your changes result in enough points, you will be asked to re-activate windows. (But, that would not take you to a site where you buy Windows - it would either re-activate automatically, or give you a phone number to ring.).