If Windows won't activate, it will generate an Installation ID number. That is the number Microsoft will require if you have to activate over the phone. Once they verify the install ID is valid, they give you an activation code to enter and your copy of Windows will be activated.
Chances are though that if you haven't done more than 2 or 3 activations within the last 6 months, you'll be able to successfully activate Windows without having to phone MS. I recently did a complete upgrade with a product key that I've had for some time. (New motherboard, CPU and hard drive). Windows activated without issue.