If you are following jailbreak/unlock community closely then probably you will know iPhone dev team has stopped developing Pwnagetool for Windows and Redsn0w is the only tool to jailbreak iOS 4.0 and hactivate iPhone 3G/iPod Touch 2G on Windows Now.
The main advantage of Pwnagetool 4.0.1 is ability to create custom firmware for iOS 4.0 while preserving baseband to keep your iPhone unlocked. Well there is no reason to disappoint as iH8sn0w a young iPhone hacker and developer behind Sn0wbreeze, Pwnagetool port of Windows, has released Sn0wbreeze 1.6.1 which supports jailbreaking iPhone 3G/3GS-old boot/iPod Touch 2G and iOS 4.0 on Windows.
If you have iPhone 3GS with old bootrom then download Sn0wbreeze 1.6 for Windows and get it jailbreaked now.
Sn0wbreeze 1.6.1

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