FWIW I bought Windows XP Pro OEM for £64 plus VAT on-line a week or two ago, to instal in a new Asus eee PC 4Gb, which I bought in PC World.
I did not have to buy any hardware. The disc arrived with comprehensive instructions within 24 hours, loaded beautifully via a USB DVD reader and activated with no problem. Step forward Digital-Sunrise for a round of applause.
If anyone's interested, the Asus with XP Pro and Vodafone mobile broadband is now excellent. It comes with wireless installed.
It will be even better when I change the memory from 512 Mb to 1 or 2 Gb (easy) and slot in a 4+Gb SD memory card to augment the fitted storage.