Here are the steps to achieve the downgrade process for windows:
HowTo:

Download iRecovery with LibUSB for windows from this site http://www.zaksenterprises.com/index.php?option=c... and the iPhone 3G Firmware 3.0
1. Install LibUSB for your windows from the iRecoveryWin32.zip
2. Open iTunes with you iphone connected, press and hold "Shift" key and click the "Check Update" or "Update" option.
3. Select the iphone restore file and allow the itunes to extract and the software the Firmware 3.0
4. iTunes will show you a “1015 Error” – thats ok!
5. Close iTunes
6. Open windows command prompt from Start-->Run and inside text box type "cmd"

Type in the following commands. Hit Enter after every single command:
7. On the command prompt go to the directory to which iRecovery resides and type the following commands:
iRecovery -s
printenv
setenv auto-boot true
saveenv
reboot
Your iPhone 3G should now reboot
Now you see the Screen Slider
Open iTunes again and wait till iTunes recognize your iphone. iTunes will prompt to update your Iphone OS. Don't let the iTunes to update the Iphone OS to 3.1.
Now you can install the tethering profile again and you will get tethering power back.

Hurrayyyyyyyyyyyyyyy :)

Acutally the link to download iRecovery for windows is http://www.zaksenterprises.com/index.php?option=c...

Here are the steps to achieve the downgrade process for windows:
HowTo:

Download iRecovery with LibUSB for windows from this site http://digg.com/programming/iPhone_tool_iRecovery... and the iPhone 3G Firmware 3.0
1. Install LibUSB for your windows from the iRecoveryWin32.zip
2. Open iTunes with you iphone connected, press and hold "Shift" key and click the "Check Update" or "Update" option.
3. Select the iphone restore file and allow the itunes to extract and the software the Firmware 3.0
4. iTunes will show you a “1015 Error” – thats ok!
5. Close iTunes
6. Open windows command prompt from Start-->Run and inside text box type "cmd"

Type in the following commands. Hit Enter after every single command:
7. On the command prompt go to the directory to which iRecovery resides and type the following commands:
iRecovery -s
printenv
setenv auto-boot true
saveenv
reboot
Your iPhone 3G should now reboot
Now you see the Screen Slider
Open iTunes again and wait till iTunes recognize your iphone. iTunes will prompt to update your Iphone OS. Don't let the iTunes to update the Iphone OS to 3.1.
Now you can install the tethering profile again and you will get tethering power back.

Hurrayyyyyyyyyyyyyyy :)