Reference: U.S. Microsoft Product Activation phone support: 1.888.571.2048
Microsoft's Product Activation phone support number is impossible to find. All Microsoft pages refer you back to your computer (Control Panel, System) but that hardly works when the computer's Activation is awry. It took me several hours to find this number.
US Product Activation (Windows 7) voice number: 1.888.571.2048
When installing Windows 7, I recommend waiting to Activate your product for as long as possible. This way, if something blows up (while fiddling with obscure drivers, for example), you do not have to call Microsoft and beg for your license again.