Try a look on e-bay you can get a full copy of win xp home oem version for about £60 genuine with activation key.you will have to check your system will run xp and back up all your files ,as it will be a new install.
Please be careful with ebay most of the regular traders are genuine but I once bought XP Pro but after six months it turned out to be a trial version yet no sign on the paperwork/activation apparent genuiness of the CD during installation
dabs amazon pcworld microland most computer web sites will have upgrades to XP which are normally cheaper than OEM versions sometimes you have to buy a piece of hardware to obtain XP eg a stick of ram to make it legal OEM version albeit you lose your warranty from the people who supplied your PC as soon as you open your PC do not take the 1st you find shop around the websites for the best price pricerunner & kelkoo will give you a good guide to the prices if you search that way
I would get an upgrade package. Similar price to an OEM, but with full m/s cover. You just need your old Windows disk for the verification. It's as well to check on M/s website that your old system can be upgraded however.
I recently bought an OEM copy of XP Home for £59 for my sons second user notebook, along with a Firewire card for my own PC £9, total outlay £68.00 I believe an upgrade would have cost around £169.00 for XP home and £259.00 for XP Pro. So my advice would be to go down the OEM route and treat yourself to a nice upgrade as well
I have plumped for an XP Home, OEM edition, at £58.81 incl. postage... as 'speed is of the essence'. Is this version fully supported?, e.g. will I be able to get MS service pack upgrades etc. (Which is why I have so many problems with installing new kit -- e.g. old drivers etc.). Also, I don't have the original MS 2000 professional install disks as I inherited the PC with op. system installed.