Just download the one-click tool, install and activate AirPrint for Windows 32bit or 64bit. Then you have to have the printer in System Preferences (printer properties) to share. Already, all printer under Windows 32-bit or Windows 64-bit for IOS 4.2.1 and AirPrint available. Please note Step 3: AirPrint.exe must be enabled in Windows Firewall.
The tool can be downloaded from here