How to Jailbreak the iPhone 3GS on Windows!
This video walks you through how to jailbreak an iPhone 3GS using a Windows operating system. Warning: This tutorial only covers the 3.0 iPhone firmware on the iPhone 3GS. If you are using a different version of the iPhone or a different firmware version, this video is not for you. The author also states he does not know if this process works on Vista, as he's tried it on Vista and has had problems. He demonstrates using Windows XP.
Jail breaking allows you customize your phone by allowing you full control over every aspect of the phone, including backgrounds and using non-Apple Certified apps.