Finally, The Chronic Dev Team has released Windows version of GreenPois0n untethered jailbreak tool for iOS 4.2.1 meaning that now you can get an untethered jailbreak on your iPhone 4, 3GS, iPod touch 2,3, 4G and iPad via your Windows PC.
There is no difference in the instructions for jailbreaking iOS 4.2.1 untethered on Windows, you can follow our guide to jailbreak iOS 4.2.1 untethered using Greenpois0n RC5 for the above mentioned iDevices.
Download Greenpois0n RC5 for Windows.