As promised, Chronic Dev Team has just released Greenpois0n for Windows to untethered jailbreak for iOS 4.2.1 for iPhone 4G, 3GS, iPod touch 4G, 3G, 2G and iPad.
You can follow our Mac guide, it's the same exactly, check the full step by step guide posted here.
Download GreenPois0n for Windows [Mirror]
Download iOS 4.2.1 for iPhone 4, 3GS, iPod touch 4G, 3G, 2G and iPad