you can use it over and over again, but you are only allowed to use it on one computer per M$. it is fine if you have to reinstall of something like that, then you have to use the key to get back up and running. however, you cannot use the key on your machine, then let your friend use it on his machine. but if I read the question right, you are asking if the key on pricewatch is a one-time-use type of thing. I would say not, and as long as only your friend is going to use it on his machine, it should be valid if he makes a hardware change and has to reactivate, or needs to reinstall for some reason.
hope this helps :beer: