Where do you live (Country)?
What is your budget?
Do you have any experience building your own computer at all?
If you share these things with us I would be happy to spend a little time out of my day to try and find you something that would work for you. If you go Prebuilt computers (Dell or HP) I have CD Disc Images of OEM windows Re-Install discs for both HP and Dell I could send you copies of (For Windows XP). I also have copies of the offline install packages for every Windows XP service pack too I could send you. You could download at work and then go home and install them on the computer via flash drive. This is completely legal as these are the original discs that shipped with the computers when they were new. The CD Keys are usually stored in the hardware and will be read by XP Setup when you install the Operating System again. Also this would be easier to get drivers as these two companies tend to still have the old XP drivers available on their websites today.
I suspect that most computers that came with Windows XP did not have key built into the hardware. For example, my old HP Pavilion laptop had the Windows XP Home product key on a Certificate of Authenticity label. The label was located on the bottom of the laptop. Note the Windows XP Home CD that came with the laptop has a different Windows XP product key in a file called unattended.txt. This is a generic Windows XP Home product key. If the CD is used to install Windows XP it will expire in 14 days. The actual product key from the label must be used to keep Windows XP activated beyond that.
A family member had a Compaq desktop that needed Windows XP Home reinstalled. I used the Windows XP Home CD from my laptop to reinstall Windows XP Home. Once that was done I booted the computer and changed the generic product key to the real one for their computer.
I have the following laptop:
Product number: Pavilion zv5200 (PM005UA#ABA)
Service tag: zv5380us
HP no longer has any support for that laptop including downloads. Fortunately, I downloaded the software and drivers for it when they were still available on their website. I am sure that is true for a lot of old computers that came with Windows XP. That means a person would have to get lucky and find the drivers and other software elsewhere online.
A person might have better luck with a non-oem computer. That is, a computer made with non-propritory parts. For example, I have a couple old computers non-oem motherboards. The motherboard are ASUS A7N8X-E Deluxe and ASUS A7V400-MX. Some drivers are still available for these motherboards. Note, ASUS no longer provides direct support for these motherboads from their website. For example, doing a google search for "A7V400-MX drivers" will find the ASUS web page for the downloads for that motherboard. I suspect support for other old motherboards from other manufacturers may be found that way.
Drivers for many old video cards with chipsets by nvidia and ATI are still available. For example, for the twenty year old nVidia GeForce3 Ti 200 video card nVidia has this driver:
Version: 93.71 WHQL
Release Date: 2006.11.2
Operating System: Windows XP, Windows 2000
Computers made between 2001 and 2006 are guaranteed to support Windows XP. ALthough most computer made in 2007 came with Windows Vista some computers were still available with Windows XP for a year or two after that.Computers with non-oem motherboards may support Windows XP several years after 2006. For example, I bought a motherboard in 2011 that still had drivers for Windows XP.
All this shows is that with a little knowledge and persistence the drivers for many old computers running Windows XP are still availble. Unfortunately, a person has to do more work now since the drivers are getting harder to find.