The following are instructions on how to jailbreak your iPhone 3GS or 4 on iOS4.3.3 with Redsn0w on Windows.
The complete guide after the break.
Before updating your phone to 4.3.3, you need to keep in mind that an unlock is not yet available for the baseband on 4.3.3. To unlock a phone on 4.3.3 you will either have to update using a custom firmware which doesn’t update your baseband or you will have to update to the 06.15 iPad baseband if on the 3GS. While 06.15 may render the phone unlockable, updating to it comes with its fair share of problems such as:
1. There is no way to come back down from 06.15, and there’s no hiding the baseband version from Apple. You’ll be voiding your warranty in a very obvious way.
2. If some future baseband comes out with a critical fix, you won’t be able to update to it if it remains down in the 05.xx sequence (then again, you wouldn’t update to it if you wanted to keep your unlock anyway).
3. Starting with FW 4.2.1 if you have 06.15 on your iPhone you won’t ever be able to restore to stock firmware (it will fail). You’ll need to only restore to custom IPSWs (then again, if you’re an unlocker you should already be doing that).
4. There are reports that 06.15 may disable your GPS. The iPhone Dev-Team is currently working on a fix.
Tools You’ll Need:
iPhone 3GS 4.3.3 Official IPSW. Click Here.
iPhone 4 4.3.3 Official IPSW, Click Here.
Redsn0w 0.9.6rc16. Click Here.
Step One
Download the iPhone 3GS or iPhone 4 official IPSW (Depending on your phone). Download and extract Redsn0w. For convenience, place all of them in a folder on the desktop, like so:

Step Two
Connect your iPhone to the computer and launch iTunes. When the phone shows up in iTunes select it from the list of devices on the left. Now hold down Shift and press the restore button. You can also choose to update but restore is recommended; it’s always better to get a fresh start.

Step Three
As soon as you press Shift+Restore, a window will pop up asking you to select the IPSW file to be used. Point it towards the Official 4.3.3 IPSW you placed in the Jailbreak folder on the desktop.

Step Four
Once iTunes has successfully flashed your phone to the desired firmware, navigate to the Jailbreak folder on the desktop and launch Redsn0w.exe.
**Windows 7 users may need to run RedSn0w in Vista compatibility mode. You can do this by right clicking the executable and selecting Properties from the contextual menu
Once redsn0w opens, click the browse button.

Step Five
A windows will pop up asking you to select the Official IPSW file for your desired firmware. Point it towards the Official 4.3.3 IPSW you placed in the Jailbreak folder on the desktop.

You will then be asked (if using a 3GS) if your device is a newer “fixed” model. If your’s is, then press the Yes button, click the No button otherwise. If you aren’t sure then simply press the Yes button.

Step Six
Once the firmware has been successfully identified click Next. Redsn0w will then prepare the jailbreak data.

After the data has been prepared you will be shown a menu where you can select different jailbreak options. You may select specific options depending on your needs, but for now we will only select “Install Cydia”. Select that option and then hit next to continue.

Step Seven
Connect the iPhone to the PC and then proceed to switch it off. Once the phone is fully turned off click Next. Remember to hold the phone in your hand before you press next as you will have to press various buttons on the phone to make it enter DFU mode.

Step Eight
After pressing next, instructions to enter DFU mode will appear. To enter DFU mode, hold the power button for 3 seconds, then without letting go of the power button press the home button and power button together for 10 seconds after which let go of the power button and continue to hold the home button until your PC detects the phone. It may take several tries to get into DFU mode.

Once the phone has entered DFU mode, the process will begin. The phone will reboot.

When it’s completed it will show you this message. The phone will then reboot and the rest of the process will continue on your device.

When your iPhone finishes rebooting (5 minutes or so) it will be jailbroken with Cydia on the SpringBoard. Your device will now be Jailbroken on 4.3.3.

Feel free to leave any suggestions or queries in the comments below.