Years ago you could just buy a keyboard or whatever and also buy a copy of XP OEM, then they changed it to something critical - ie, CPU, Hard Drive, etc. In recent years since just before vista's release they changed it to a whole package, so at my work we can only sell it with a new system or laptop, or we install it for you. But for some certain customers we can make some form of deal - some forum members have already seen me about the deals.
MSY imports all/most of there stock from China (grey imports), so you dont get any Australian warranty, but with software it doesn't matter, all you need is the CD Key. As the stock comes from China, they don't have to conform to the licensing of windows as much as we do, as our's is Australian stock.
Also be VERY careful of the ebay jobs, we have seen quite a few customers come in with copies of XP OEM they bought off of ebay - High Quality Fakes, it cost then about $120....then they came to us and it cost them about another $200 (cost of XP oem & install fee).