trying to put xp pro on, but after formetting pc seems to remember old product key

my friend is trying to install xp pro, he is replacing a pre installed version of the same, what is happening is that after formatting xp wont recognise the product key on the new xp pro,i know this is correct,it seems to be saving the key from the version before formatting. even after formatting it still remembers the old key and wont accept a new one for the new version , is xp creating a hd partition to save the old key to ??? we cant find out how you check whats on each partition either.......... what i need is to be able to totally erase all trace of the old key no; and re install my new version and new key.....how can it remember old key no after formatting
thankies for help

 

There are different keys for various "versions" of XP. The Volume License, Upgrade, Full Version, and OEM all have different key series. If the hard disk is clean, and you have the key for the CD you're installing, it should work. However, even though you have two XP-Pro CD's, they may be different "versions" with different key series.