Hi breadman - I don't know if you are still watching this, but here goes.
Microsoft have an informative page on how to buy, including how to get a cheaper "upgrade edition" and also how to purchase an additional licence.
Could I ask why you need XP? Windows 2000 runs most things even these days, and depending on your needs you may wish to save some money now (and some hassle later on) by trying Ubuntu Linux.
I use Linux, and others here do also. Just done a search here for more info, and there is a thread called Linux and Daytrading which is relevant.
I'd panick about running anything financial on Windows myself. For those of you wanting to try Linux, it can do most stuff MSFT software can, with the exception of 3D apps and some software. CMC MarketMaker, for example, doesn't work on Linux. I keep Windows 2000 around in an emulator which can be run from within Linux. I find Linux faster, more reliable, and more stable than Windows. It is not for everyone, but Windows "power users" should do fine with Ubuntu. Interactive Brokers, and all the web spread betting firms (with the exception of CMC) work with Linux. Before I needed CMC MarketMaker, I had gone without any form of Windows for a considerable time.
Most traders here I think will have a trading machine and a machine for general use (this is only wise - financial stuff should be kept separate and I think most traders here can afford a second machine - thats 40 pips to enough folk here). The Linux Emporium sell Lenovo (the company which bought IBM's PC division) laptops for around £400 upwards, and they are pretty good machines. Great company too - I just bought a laptop from them. Perhaps something to think about when buying a new desktop or laptop - save a bundle on the Microsoft Tax.
Hope this helps - let me know if there is anything else.