Normally,Windows CAN be activated as many times as you like on the same hardware.
Retail versions can also be moved to new hardware.
However, that doesn't mean that all activations can be done over the internet.
Usually Retail versions can be activated 10 (I think) times before telephone activation is required - although that is not a fixed number and can also depend on the frequency of activation. OEM versions get 5 activations before hitting the trigger-point (I believe)