Another way to get XP cheap is to buy the OEM version. Just buy some cheap hardware to upgrade your computer, and get the OS with it. Here the OEM version is about $10 cheaper than the upgrade version.
Some companies also have a deal with Microsoft to buy their software at discounted prices. I picked up Office 2003 Pro for $260 CAD, which is half the price of the OEM version and far cheaper than the retail version.
goates