These are instructions on how to jailbreak your iPhone 4, 3Gs on the 4.3.1 firmware using RedSn0w for Windows. The Mac version of this tutorial is here.
Make sure not to update to iOS 4.3.1 if you need an unlock as one is not yet available!
Step One
- Download RedSn0w from here
- Download iOS 4.3.1
* iPhone 4: 4.3.1 (4 GSM): iPhone3,1_4.3.1_8G4_Restore.ipsw
* iPhone 3GS: 4.3.1 (3GS): iPhone2,1_4.3.1_8G4_Restore.ipsw
Extract the RedSn0w zip file.
Step Two
Launch the redsn0w application from the redsn0w folder we extracted earlier.
Step Three
Once RedSn0w opens click the Browse button and Select the 4.3.1 firmware ipsw then click Open. 
Step Four
Once the firmware has been verified click the Next button to continue.
Step Five
RedSn0w will now prepare the jailbreak data
Step Six
From this window you can select the jailbreak options you would like.
Make sure Cydia is selected and click the Next button to continue.
Step Seven
Please plug your iPhone into the computer and make sure its OFF then click the Next button
Step Eight
RedSn0w will now guide you through the steps to get into DFU mode.
Hold down both the Home button and the Power button for 10 seconds.
Release the Power button and continue holding the Home button until RedSn0w detects the device.
Step Nine
Your iPhone will now reboot
Step Ten
RedSn0w will then begin uploading the new RAM Disk and Kernel.
Step Eleven
Once this is complete you will be notified that RedSn0w is done. Click the Finish button. When your iPhone finishes rebooting (5 minutes or so) it will be jailbroken with Cydia on the SpringBoard.
*As usual a big thanks to the iPhone Dev-Team and I0n1c for their hard work and contribution to the iPhone community.
**Windows 7 users may want to try running the redsn0w executable mode as an administrator in Windows Vista compatibility mode if you experience any trouble.
