I think you may have trouble installing the XP upgrade on a new machine. Try instead getting XP OEM from click here for around £55.
Alternatively click here will build you a custom machine to your specs for £1060 delivered. Although I have built my own PC in the past, I didn't get any enjoyment from it and actually found it very frustrating and with a pre-assembled PC you do get a warrenty. I can't tell you much about the company/service, I ordered a PC from them a fortnight ago and it is due for Delivery on Wednesday. They have responded to both telephone and email enquires so that's a positive.