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Download redsn0w 0.9 Beta 3:
To access your iTunes IPSW on Windows, open up Run and enter the command %AppData% Open up Roaming/Apple Computer/iTunes/iPod Software Updates (the folder will be iPhone Software Updates if you are using an iPhone). Be sure to copy (not move) the file to a safe place, as your IPSW is a very handy thing to have when jailbreaking.
If you are having problems finding the IPSW, alternatively you can download your respective firmware from this website: /
If you are using an iPod touch, to check to see whether you have an original 2nd generation or a newer one, open up the Settings application. Click General and then About. Look at the Model Number. If it begins with MB then redsn0w will work without a problem, but if it starts with MC then redsn0w will run a tethered jailbreak. You MUST tell redsn0w when it asks if the Model Number starts with MC when the prompt appears. If you answer wrongly it could cause you to restore your device and do it again.
A tethered jailbreak works the same, except that when your iPod shuts off you will need to plug it into your computer and run redsn0w for it to start. If you are using a 3rd generation iPod touch (the new models with 32GB and 64GB capacities) then redsn0w won't work at all at the moment. I would recommend using blackra1n for a 3rd generation iPod touch.
The jailbreak works much the same way for a late model iPhone 3GS. In that case you will get a prompt explaining the correct way to find out whether your 3GS is a newer model or not. Same goes for this: definitely get the question right or you will have to restore. If you have to do a tethered jailbreak, the same rules apply: if you need to shut off your device, to reboot it simply plug it into your computer and run redsn0w and you will be good to go.
If you have any problems don't hesitate to either comment or send me a PM.