If you activate it on the current computer and then want to put it on a new computer, you will need to contact MS and get there blessing. You will need to let them know (in a convincing manner) that you will not be using it on any other computer.
If you activate, then reformat and reinstall, you will need to reactivate, but you may be able to do that without contacting MS if you wrote down the activation code they gave you when you activated. You will not be able to use that code to activate on another computer. MS REALLY doesn't want you using it on more than one computer.