Is a Certificate of Authenticity (COA) only valid when it's put on a machine

[Manuel]

Hallo,

let's say I have a computer and bought a full version of WIN XP. Do I
have to stick the COA on the computer or is the license also valid
without sticking it on the computer.

What I have in mind:
I want to sell the computer and keep the license for myself and use it
for another computer.

I know that it is not possible for a OEM version, but can I do it with
a full version? Is a COA valid without sticking it on the box.

Thank you in advance
Manuel