I was recently looking into having iOS devices print to Windows Print Server using the AirPrint feature, in order to do so I found a number of articles to install third party tools to achieve this.
Firstly, I needed to install the Bonjour Print Services from http://support.apple.com/kb/DL999 and if required enable inbound connections on UDP service port 5353.
Once this is installed, you will need to install the AirPrint component from http://cdn.tweaks.com/dl/article_mirror/airprint_installer.exe.
Once downloaded run the executable, and select Install AirPrintService and set to the Startup Type to Automatic.
Authentication may be handled by using the domain account of the user, or by granting the guest account access, this will require you to enable the local guest account on the print server.
I had one issue which was fixed by downloading registry entries and following the steps at http://forums.macrumors.com/showthread.php?t=1293865, for iOS devices that are version 5.x and higher.