How to Jailbreak Your iPhone 3GS, iPhone 4 Using Greenpois0n (Windows).
Here are instructions on how to jailbreak your iPhone 3GS and iPhone 4 using Greenpois0n for Windows. Greenpois0n has been updated to support the iOS 4.1 and iOS 4.2.1 firmware. Use Limera1n if you want to jailbreak iOS 4.0, iOS 4.0.1, iOS 4.0.2.
Open your web browser of choice and navigate to http://www.greenpois0n.com. Click the Windows link at the bottom of the screen.
When prompted save the download to your desktop
Double click the downloaded greenpois0n-osx_rc5.zip file to extract the archive then double click greenpois0n from the desktop to launch the application
Connect your iPhone to the computer, power it off, then click the large Jailbreak button
You will now be instructed on how to place your iPhone into DFU mode.
press and hold the sleep button for 3 seconds.
Continue holding sleep; press and hold home for 10 seconds.
You will then be prompted to release the Sleep/Wake button but continue holding the Home button.
Once your iPhone is successfully in DFU mode the jailbreak will begin. You can release the Home button. Click the Complete button to close the application.
After the iPhone restarts you’ll notice a new Loader application on your Springboard. Press to launch it.
Press to select Cydia then press the large Install Cydia button that appears.
Once Cydia has installed successfully you can remove Loader by pressing the Options button then the large red Remove Loader.app button.
Restart your iPhone then run Cydia. Let it do all the necessary updates then follow our Unlock tutorial if possible for your device.