XP disc types and license

[Dr Zoidberg]

Hi

I have purchased a branded PC for a relative, it was the most sutiable thing to do for them for what they wanted and cost wise.
It came with Vista on it and a branded XP disc to downgrade,
The box has a vista label with vista key on it,
The XP downgrade does not ask for the key during instal, but can be found once installed.

The branded downgrade disc also installs all the branded software and junk they seem to think needs to be on there with out options to not instal it.

I would like to use a clean windows only disc with the legal key on the box to give it a clean instal with out the junk added.

Im guessing the XP on the branded disk is corp version? as it does not require activation.

Can i use this key with an OEM disc?
If so will it require activation? at which point im guessing that wil fail online?
If so could i phone them and say 'can i activatre this corp key please?' at which point i would expect to get a 'No' ?

Is there any other way to do this?
Or is my only option to use the branded disc, then uninstal the junk and try and give everything a good clean?

Thanks