Now when iTunes 9.2 for Windows is officially out, you can install iOS4 GM using Windows as well. We have already covered how to install iOS4 using Mac, here is the step by step guide for Windows:

Before you Begin:

Make sure that you are not using an unlocked device (using blacksn0w, ultrasn0w or yellowsn0w) as iOS4 will update your baseband.

IMPORTANT: Baseband 05.11 and 04.26 unlock solutions are available. You will need to preserve your baseband while updating to iOS4 if you want to unlock your device. If you want to preserve your baseband, DO NOT follow this guide. Instead, jailbreak iOS4 with PwnageTool and then load custom IPSW into iPhone.

Downloads:

Step 1: Download iTunes 9.2 for Windows

Step 2: Download iOS 4 GM candidate for iPhone 3GS, iPhone 3G, iPod Touch 3G, iPod Touch 2G.

Installing iOS 4:

Step 3: Install iTunes 9.2 on Windows.

Step 4: Connect your device to computer and Sync with iTunes.

Step 5: Switch to iTunes, hold “Shift” key and click on “restore” button.

Step 6: Locate and select iOS 4 IPSW file. Now let the iOS 4 transfer complete.

DISCLAIMER: We strongly recommend not to download iOS 4 torrent as its neither legal nor recommended way to upgrade to iOS4. Guide posted here is purely for information purposes only and if anything goes wrong with your iDevice during/after the process, be ready to hold yourself responsible for that. Official iOS4 update is coming on June 21 anyway.