That's just an update...that's not an actual SP3 disc.
And have you had XP on that computer before? You can't just use an XP key from another computer, as that's illegal. If you already have XP installed, google for Belarc and you should be able to find your key that way. If you don't have XP installed already, then you'll have to find the sticker that has your XP key on it. If you've had XP on that computer before, they would have been required by law to put the sticker on there.