The Chronic Dev-Team recently released windows version of Greenpois0n which brings an untethered jailbreak to all iOS 4.2.1 devices.
I test it on my iPhone 3GS and iPad (WiFi) and it works well for me. I use both Mac and windows version of Greenpois0n and it works well on both. I heard some rumors that it can also jailbreak Apple TV 2G running iOS 4.2.1.
Although Greenpois0n guided you to boot your Device in DFU mode, you should have some basic info about DFU mode -Read more-.
You must do a clean restore before using Greenpois0n
Download Link
