These are instructions on how to jailbreak your iPhone 3GS or iPhone 4 on iOS 5.0.1 without updating the baseband using RedSn0w for Windows. You may want to do this to preserve an unlock from a lower baseband version.
Please note this jailbreak is tethered for every device but the old bootrom iPhone 3GS which means you will need to connect your device to the computer each time you reboot. Also, it may be prudent to save your SHSH Blobs.
If you want to jailbreak a lower firmware version you can find the appropriate tutorial here.