If you have used sn0wbreeze to create a custom IPSW file and have no idea how to restore in iTunes, heres a complete how-to guide for Windows!

1. Download sn0wbreeze 2.X (X = 3,4,5 or 6 and above!)
2. Open sn0wbreeze, on Vista and 7 make sure to run as administrator.
3. Near the top left of the program should be some buttons (File, Help and iREB), click on iREB.
4. You will then be taken to the iREB screen. Plug your iDevice in via USB and turn it off. (Hold power for 5 secs then slide to power off)
5. Follow the onscreen instructions to enter pwned dfu mode.
6. Once you have entered pwned dfu mode, close sn0wbreeze and open iTunes.
7. It will say your device needs to be restored, but do not press anything.
8. Hold Shift while clicking on restore, if all went well, you should be prompted to select an IPSW file.
9. Select the IPSW file that sn0wbreeze made for you.
10. iTunes will wipe out all data and restore to 4.3.X Untethered, jailbroken. (Unless you used sn0wbreeze 3 or 4, which is tethered)

Also, for anyone interested, http://www.felixbruns.de/iPod/firmware/ is the absolute mother load of ipod/iphone/ipad firmware. I have used it before and it works beautifully. It should work with sn0wbreeze, although I haven't tested it myself.

If you need any ios firmware files, the ipswdownloader app for windows and mac works great. Just google ipsw downloader and you will find the link to download it. Fastest way I've found to get any ipsw that you need.

If you use that in conjunction with jailbreakme.com you've got yourself an untethered jailbreak.