if you are reinstall xp on a dell system with a dell disk you do not need the key.

To late to ask for it, that system was sold a while ago, probably when new they (Dell) probably will help.

As Boppo said using Dell Disk on Dell computer the COA lable is useless, If you will use a diferent disk (retail) that should have its own key, and the Dell one wouldn't work.

As noted, you don't need it for a Dell OEM reinstall.  But, if you still  want it, download the Belarc advisor  www.belarc.com  Run the advisor and it will profile all your hardware and software and list the Key's for many of your software including Windows.