I think most online computer parts retailer will sell you the full version of Windows XP OEM for around $150 if you buy at least one hardware item like say a CDROM player.
Quote from <A HREF="http://www.googlegear.com/ggweb/jsp/ProductDetail.jsp?ProductCode=604637" target="_new">GoogleGear</A> site:
Under Microsoft’s Distribution and Preinstallation Agreement, computer stores such as Googlegear are only allowed to sell a Microsoft OEM hardware/software product in conjunction with another hardware item. The agreement does not specify what type of computer hardware, so you <b>may choose anything in our Hardware or Systems departments -- even a cable</b>. No software, or CD/DVD/zip media.