I got so good at reinstalling Windows XP and then calling the Microsoft number to re-activate. Most of the time the automated system worked, but sometimes you even had to talk to a human and explain why you need a new key.
Assuming the OP didn't have a reliable internet connection you would need to call in phone number to activate.
I wasn't allowed online until I turned 16 and was able to get my own job and pay for a second phone line. Prior to that, I had to shuffle stacks of floppies back and forth to use the T1 in my high school.