This guide will show you how to jailbreak iOS 4.2.1 on iPad Wifi or 3G model using the Greenpois0n tool on your Windows Computer. As you might have already heard Greenpois0n update has been released for Mac and Windows to jailbreak any device running iOS 4.2.1.
For Mac Version of this tutorial read How to Jailbreak iOS 4.2.1 on iPad using Greenpois0n [Mac]
Please note that Greenpois0n does not update your firmware and you should first upgrade your firmware using iTunes first. In order to upgrade firmware download the .IPSW firmware file from here and then click Shift+Restore in iTunes to update iPad. If you don’t know what your current firmware version is or how to find iPad firmware version read How to Find iPhone Firmware and Baseband Version.
Steps to Jailbreak iOS 4.2.1 on iPad:
Point your browser to www.greenpois0n.com and click on the Windows link to download Greenpois0n tool.
After you save the archive and extract the files run Greenpois0n.
Once you have upgraded your iPad to iOS 4.2.1 as instructed above connect it to your computer using the USB cable and quit iTunes in case it is open. Power off the iPad by holding the Power button and then sliding the on screen slide to power off button.
Click on Prepare to Jailbreak. Now press the buttons on your iPad as instructed on the screen to put it in the DFU mode. You can keep trying till you do it right. No need to panic if you fail at the first attempt.
Once the iPad has entered DFU mode click on Jailbreak.
After Greenpois0n jailbreaks your iPad click on the quit button.
After your iPad restarts you should be able to see a new Loader icon on your homescreen. Tap on Loader to launch it. We will use it to install Cydia on iPad.
Tap on Cydia and then tap on Install Cydia. Post installation press options and then tap on Remove Loader.app to remove this app which is not required any more.
You now have a Jailbroken iPad running iOS 4.2.1 with Cydia installed. Peace!