changing product key

I have a system unit (Note book)that the original restoration disk (OEM) that came with the system had got missing and now the Harddrive is bad that i have to replace it . I got a new HDD and installed an Operating System - Windows xp sp2 with Volume Licence Key, now i want to change the key to real one that came with the system because i have the COA being pasted at the back of the system unit, i did all i could but i still couldnt change from the old key (vlk).

please could there be any mistake im making in the process ? I need somebody to put me through.