You did activate it, but by registering you can get replacement media for cheap (compared to the alternative.)
I have been reading about sometimes having to re-install XP. In order to be prepared, I was making up a package with floppies and find my XP upgrade CD has gone AWOL! The package is there along with the serial number, no CD
I know, I should have bought the more expensive one, but that one would be AWOL now too!
If, God forbid, I ever have to re-install, any suggestions?