OK go here iClarified - iPhone - How to Jailbreak Your iPhone 3GS Using RedSn0w (Windows) [4.3.1] that is the iclarified instructions for redsn0w. You are already running 4.3.1 so start at Step Three. It is like I said above you don't need to do anything with iTunes. You just run redsn0w point to the 4.3.1 ipsw that you download and choose your options. It will then lead you through the steps to put the phone into DFU mode and redsn0w will load up the patched firmware onto your phone.
OK I would say try to download it again. I know you're on Windows so I wonder if it's some kind of issue with how it downloads the file? When I get home if you haven't fixed it yet I'll download it for you and put it on my dropbox and see if that helps.
It totally slipped me that I didn't brin my laptop home. I tried to use my GF's but she was too busy with it. If you still have the problem tomorrow I'll send you the link to the file on my Dropbox.
I've downloaded the files and proceeded to jailbreak my iPhone 4 and I get all the way to where it says it's downloading ramdisk and it seems like my computer freezes or something because it stays on that part of the process for what feels like forever. Does uploading the ramdisk take long or is there something I'm doing wrong? Any help is appreciated. Thanks!