DISCLAIMER: This post is purely for educational purposes only and is subject to faults or imperfections. TechPinas can NOT be held responsible for any damage that this jailbreak procedure may cause on your iPhone 3GS. I suggest that you read and consult other jailbreak procedures before proceeding. Also, keep in mind that jailbreaking any iDevice will void its Apple warranty and make it vulnerable to viruses and other attacks. If you're OK with these conditions, proceed with utter caution.
Jailbreak iPhone 3GS 4.1 using Snowbreeze 2.0.2 for Windows Procedure:
1. Check if your iPhone 3GS is supported with Sn0wbreeze V2.0.2 for Windows (NOTE that this procedure can not be done using a Mac.)
iPhone 3GS with New Bootrom (iBoot-359.3.2) must already be previously jailbroken on 3.1.2. On the other hand, iPhone 3GS with Old Bootrom (iBoot-359.3) must also be already jailbroken and if jailbroken using JailbreakMe or Spirit, install Spirit2Pwn first via repo.woowiz.net.
2. Download iTunes 10 via the link.
Download Snowbreeze 2.0.2 for Windows.
Download iOS 4.1 firmware for iPhone 3GS here.
3. Back-up iPhone 3GS using iTunes 10.
Preparing the Jailbroken IPSW
4. Open Sn0wbreeze version 2.0.2
Make sure your PC has .NET Framework 3.5 installed. If not, download it via the link, then install on your computer.
5. On Sn0wbreeze, hit OK button and then, Next.
6. Choose Expert Mode and click Next.
7. Hit Browse button and select the IPSW (iPhone Software) iOS 4.1 firmware file that you downloaded from Step 2. Let Sn0wbreeze process the file. Once done, click 'I understand' and 'Next' buttons. The custom firware file will be automatically saved on your desktop.
Installing it on your iPhone 3GS
8. Put your iPhone 3GS in DFU mode; Open iTunes 10, connect your iPhone 3GS to your computer via USB. Press and hold down iPhone's Home and Sleep buttons at the same time for 10 seconds. Then, release Sleep button but keep holding down Home button until an iTunes message pops out telling you that the program has detected an iPhone in Recovery mode. Note that an iPhone in DFU mode shows a blank screen.
8. On iTunes 10, select your connected iPhone 3GS from the left navigation pane.
9. Hold shift key and click left mouse button to recover your iPhone 3GS with the custom firmware that you made using Sn0wbreeze via Step 7
10. Let iTunes complete the restore process.
There you go.
Let me know if this procedure worked for your iPhone 3GS by leaving a comment below. Also, if you think we should add or edit something on this jailbreak guide, kindly leave a note.