Here is a quick guide on how to enable AirPrint on your Windows machine assuming you have iOS 5 or greater.
1) Go here to download the latest files that you need in order to enable AirPrint.
2) Double click on the "AirPrint_Installer.exe" application and then click on "Install AirPrint Service".
3) Run the "AirPrint iOS 5 FIX - 32Bit.reg" or "AirPrint iOS 5 FIX - 64Bit.reg" (depending on what type of os you have installed) to install the current version for iOS 5.
4) Again, open "AirPrint_Installer.exe" and double check that "Service Startup" is set to "Auto" and click "Start".
5) Restart your computer, you are almost done.
NOTE: You may need to enable the guest account on your computer by going to Control Panel > User Accounts > Manage Another Account > Turn the Guest Account on. In the AirPrint_Installer.exe, you may need to choose the guest account that you just enabled. Click "Enable Guest Account" and then click "Update." Again, you have to restart.
If you have any questions about this process, please post in the comments below!