I have done it many a time, it is the only way to re-activate it from a system reformat or whatever. I did it and you are unable to use xp after 30 days if you dont activate it. To make things worse you have to call up and talk to the stupid phone. yeah the phone, some computer comes on and asks for the installation id (now if you activate more than one os with this they will find out and make you buy a new license). Of course microsoft is too lazy to get their tech people on the phone. Then you start slowing trying to get this damn computer thing to understand you voice and then when you are done chatting in this 40 something character key it says they cant find it...so THEN you can get the tech dudes, they then quiz you over all kinds of stupid crap. Then THEY give you another cd-key and you have to put that in, then you are all done. Possibly the stupidest thing in the world.