Pricewatch has a software category now. There's a bunch of people selling something with the following description for $7:

Preinstallation CD allows you to install win. XP (Not an OS) without CDKey and customize it before you sell

I'm guessing that that's not what you want, or that it would be not much better than Kazaa if it was. Otherwise, it seems to get down to about $90 for an OEM version.