i found this some where in the internet, i dont remember the link exactly am just posting it here, hope it will help you guys.

The step by step instructions that Windows users need to follow to jailbreak iPhone 3G is mentioned below.

Step 1: You need to download two files: x12220000_4_Recovery.ipsw and iPhone1,2_2.0_5A347_Custom_Restore.ipsw.

Step 2: Make sure you have iTunes 7.7 installed.

For Vista: Click Start then copy this into the search box and press enter after:
For XP: Click start then run, then copy this into the run box:

%appdata%\Apple Computer\iTunes

Step 3: Make a directory called: Device Support. If the directory already exists, clear it.

Step 4: Copy x12220000_4_Recovery.ipsw into the Device Support folder you just created.

Step 5: You need to get your device into DFU mode. You can follow these steps to achieve this:

Step 6: Once your phone is in DFU mode and detected by iTunes (should be black screen on phone NOT iTunes logo with USB cable), click on iPhone in iTunes, then hold the SHIFT key and click on restore.

Step 7: Browse to the folder where you downloaded iPhone1,2_2.0_5A347_Custom_Restore.ipsw and select it. iTunes will then extract the file and you will see the iPhone Dev Team's pineapple logo pop up after a few seconds, as seen in the screen shot below:

The procedure should also install the Open Source Package Installer and Handler for the iPhone, Cydia which should make installing the unofficial native iPhone apps on your iPhone 3G easier.

This is for those who requested for a guide to only jailbreak iPhone 3G (without unlocking it) using Windows.