First, you install with that disc you have, using the key that goes with the disc.
Then, you use this link:
http://www.microsoft.com/genuine/selfhelp/XPPkuinst.aspx?displaylang=en&sGuid=6ed828ca-66fc-4a26-a991-c7ba283a5e54
to update your product key to the COA sticker on the bottom of the PC. A phone activation
may entail, but honest answers to Microsoft will get your product reactivated.
This is assuming you aren't using a bootlegged VLK version of XP pro on that disc.