you can get an OEM version for £60
if you prepare to do some work and also prepare to take some risks, you can make the installtion CD yourself, no big deal, all tools are freely available.
DO NOT test the produced CD on a working PC!!!!!!!!
Please backup important stuff before you try anything. You take your own risks.
(1) make hidden partition visable (files are stored in hidden partion), there are may tools for those, for example Partition Expert from Acronis or partition magic from Norton. A free version of partition expert was on PCA cover disks a few moons ago.
(2) Copy everything on hidden partion to a new location, only i386 directtory is needed actually
(3) make hidden partition invisable again, just pu back to the original situation.
(4) Download XP service pack from micorsoft
(5) Now you are ready to make your own installation CD. Again there are many tools, the simplest is nlite click here, you can also use "Bart's way to create bootable CD-Roms" click here
Important notice: Do not test the CD you produced on a working PC. This is becuase the CD produced in this way will be so called "unattended installtion CD" so it will automatically wipe everything from HDD if you boot it up from it.