One license, one computer

I know this has been asked and answered many times, but
the links are so wordy and explicit that it would be nice
to get a simple answer.

My present OEM computer came with its license, so I have
no problem, myself. I bought this new machine 2 years ago
to replace the eMachines PC that had so many intermittents
on the motherboard that I was tired of massaging it.

But it seems to me that I still own the license that applied
to that computer. My cousin is giving up her old HP desktop
computer (with Windows 98) because her daughter forced her to
accept a new Apple laptop of some sort.

I would like to take her old machine and adopt it as my own,
and use my XP License and run XP on that machine. My OLD XP one-
license-one-machine, only it's that old license handed down from
my defunct eMachines frame, now to run on a different machine.
One license, the original machine destroyed, now to be used
again, on one machine.

It sounds legal to me, but I'd like advice in plain language, as
to how I do it.

Thank you.