If it is an OEM disc - it will say this on the packaging or the disc - you cannot use it because of the pre-registration. Similarly with an Upgrade disc you would need to have a legitimate copy of a previous Windows OS installed on the computer.
If however this is a genuine "Full Version" of XP with hologram you could install it on your computer. You would almost certainly have to validate it by telephone explaining that you have bought the original disk (Get a receipt)and have the original packaging. Confirm that you have only installed it on one computer.
The only trouble with this is knowing if it has only been installed on one computer. Does the Charity Shop know where this disc came from? I suspect not, in which case there`s a chance that the CD Key is in use somewhere else. I`m not sure that Microsoft will be able to tell if they give you a new activation code. Maybe the Charity Shop might consider the situation and allow you to buy the disc on approval.