FingerPrint (OS X & Windows) is a paid application that enables a printer connected to your Mac or PC to become an AirPrint printer. It is a paid application.
There is also a free program for Windows. You can download the files here. One they are downloaded, unzip the zip file and run the
AirPrint_Installer.exe Then run either
AirPrint iOS 5 FIX - 32Bit.reg or
AirPrint iOS 5 FIX - 64Bit.reg depending on if you are running a 32 or a 64 bit version of Windows. Go back to
AirPrint_Installer.exe, make sure "Service Startup" is set to "Auto" and click "Start". Reboot your computer. All these instructions and troubleshooting instructions can be found in the links above.
AirPrint Activator is a free application that allows you to turn any shared printer into an Airprint printer. I have used it quite a bit now, and I highly recommend it. It has a very simple and easy to use UI: