You can buy OEM versions of the software. Microsoft says you must be building a computer, or something along those lines. Most places that sell OEM copies require you to buy a hardware component as well. They claim this meets Microsoft's rules. Whether that is a gray area or not doesn't really matter to me. I buy licensed copies from authorized sales agents, how they comply with Microsoft is between themselves and Microsoft.
Just so you don't get surprised, if you buy an OEM version as a so called builder then if you have a problem don't contact Microsoft. They will refer you to the builder, and you already know him. You will get full access to the upgrades and service packs, just no tech help if you have an issue.
OEM versions of XP Pro run around $140.00 in the States. They are available over here as well for a bit more, but with the dollar in the dumps I prefer to buy over there.