The issue is with pre-installed machines, or ones installed from a manufacturers recovery CD.
These CDs are mass produced for the manufacturter, and therefore identical, but will only install on the same manufacturer's PCs. They rely on a "tatoo" in the bios to prove the machine is from that particular manufacturer. This is why if you try to use a recovery CD from a Dell, on say an HP it wont work. It will however often work on an entirely different machine from the same manufacturer.
They all have the same key pre-coded on the CD. The SIDs and UUIDs once installed will be different however, as these are generated from unique features of the PC such as Mac address, serial number of CPU, system board serial number.
See here for a similar thread.
viewtopic.php?f=5&t=3091&p=13982&hilit=XP+key#p13982