I suggest that you check MS website for how XP is licensed, what the versions are and how you qualify for OEM, upgrade etc.
Caveat Emptor.
As a service for all, here's the gen straight from MS:
Important Restrictions on Pre-Installed Software
Buying pre-installed licences is a cost-effective way of acquiring software, particularly when you’re replacing hardware at the same time. But like buying an airline ticket, there are some important terms and conditions you need to be aware of before making your choice.
Transferring Desktop Operating System Licences on Pre-installed Software
Desktop Operating System software (such as Windows Vista and XP) that is pre-installed on PCs cannot be transferred; which means it lives and dies with the ‘machine’ it was installed on, unless repaired under warranty. We define the ‘machine’ as the motherboard.
Buying Software Designed for Pre-installation without a PC
Before it is installed onto a PC, software designed for pre-installation is known as OEM system builder software. You may see OEM system builder software for sale that is not pre-installed on PCs.
This usually comes in a pack and is referred to as an OEM system builder pack. This is only intended to be sold to manufacturers or system builders* who are building machines for their customers; end users should not buy OEM system builder software.
Once you open an OEM system builder pack, you are responsible for the ongoing technical support** of that software; the rationale behind this is that if you are opening these OEM system builder licence packs you will be a system builder and it is appropriate for you to be providing support.
*A system builder is defined as an Original Equipment Manufacturer, an assembler, refurbisher, or pre-installer of software on computer systems.
**This effectively means that the person/organisation that opens the OEM system builder licensing pack is responsible for resolving any issue with that software.
Frequently Asked Questions (FAQs) about Pre-Installed Software (also Known as OEM System Builder Software)
Q : Does buying pre-installed software give me a full copy of Windows?
A : Yes, purchasing pre-installed software does give you a full copy of Windows.
NB: A Volume Licensing agreement only offers Operating System upgrades; this means that your agreement doesn’t give you the right install a Windows Operating System on any newly-purchased PCs, only to upgrade machines already covered under your agreement.
Q : Can I transfer my pre-installed Windows software to my new PC?
A : No. Pre-installed software lives and dies on the PC that it is installed on.
Q : Can I upgrade the software that was pre-installed on my PC when I bought it with boxed software from a shop (called Full Packaged Product)?
A : Yes
Q : I have bought a PC with Microsoft Office pre-installed. Can I downgrade this to a previous version of Microsoft Office?
A : No. Pre-installed Microsoft Office does not have downgrade rights.
Q : I have purchased Microsoft Office pre-installed on my PC. Can I install a second copy on my laptop?
A : No. Pre-installed Microsoft Office is tied to the original PC and does not have portable use rights. These are only available when you buy boxed software from a shop (called Full Packaged Product) or through Volume Licensing.
Q : Can I install my pre-installed Microsoft Windows on more than one PC?
A : No. Windows is licensed per hardware device. This means one licence is required for each device that it is installed on.
Buying a FPP gives you the right to transfer the software to another PC, and also to MS product support (although I do not know how worthwhile the latter is).
Basically, you will not be able to re-activate your OEM software on a replacement PC. OEM is a one-shot deal. You should still be able to re-install it on the PC that it was originally installed on.
SD