Step-by-step Tutorial: How to Jailbreak and Unlock Your iPhone 3G/3GS Using RedSn0w 0.9.6b5 (Mac OS X) [iOS 4.2.1]
Jailbreak utility RedSn0w 0.9.6 b5 is available for both Windows and Mac OS X. The process is the same on both operating systems. So you can use redsn0w tutorial for windows. There is only one major difference - download redsn0w for Mac OS here or