http://jaxov.com/2010/11/how-to-enable-airprint-service-on-windows/ is out there for Windows, but reports say this stopped working with iOS 5 and iTunes 10.5, likely due to the major iTunes rewrite.

Apple was working on official support for this, and then abandoned it for some reason. Most of these hacks try and extend what Apple shipped as a developer beta. As Glenn said, YMMV with these solutions.