Printing from iOS devices to existing printer connected to windows pc?
The way suggested by @Clayton Jones above, links to a good tutorial.  However, on the same site, I found an installer that will do all of this for you.  Here are the written instructions, for after you have downloaded the installer, via jaXov. You can also find a link with full color step-by-step screenshots in the previous sentence.

If you are facing any difficulties in following the manual procedure to enable AirPrint service on Windows, now you can install AirPrint service through AirPrint installer (by TadGhostal) which is compatible with Windows 7, Windows XP and Windows Vista. For those who don't know, iOS 4.2.1 comes with a unique feature called AirPrint which allows you to print your documents, emails, web-pages and photos over a Wifi connection with your Apple iOS device.

Prerequisites:
Download and install iTunes 10.1 from here
Make sure the printer is setup as a shared printer on your local PC
Printer and iOS device should share the same Wifi connection
Make sure you have full administrator privileges to enable / disable services on your PC
Procedure:
Download AirPrint installer for Windows from here. Once downloaded, right-click on setup.exe and choose Run as administrator from the pop-up menu.

Just follow the on-screen instructions to install AirPrint service on Windows:

Once installation completes, Windows Firewall will prompt you allow or deny AirPrint features to be shared over networks. Click on Allow access button.

Please note that only HP ePrint enabled Photosmart, Officejet and Laserjet printers currently supports Apple's AirPrint service. You don't need any additional network setup if you are using one of these printers and can print any media directly from your iOS 4.2.1 AirPrint enabled iDevice.