I got stuck when downgrading from IOS 4 to 3.1.2, giving me “unknown error 13″. When I hit restore (in DFU mode), it extracts the software and when it says “Preparing phone for restore” my phone reboots and it gives error (13). This is what solved this issue for me.

If you use windows, go to start -> run -> and type %appdata%
Open “Apple Computer”, Open “iTunes”, check if there is a folder named “Device Support”.
If the folder exists, delete any contents inside, if the folder does not exist create it. Make sure you empty the recycle bin.

If you use mac, go to “user name”\Library\iTunes\, check if there is a folder named “Device Support”.
If the folder exists, delete any contents inside, if the folder does not exist create it. Make sure you empty the recycle bin.

Connect your iPhone to the computer and put your iPhone in DFU mode. Once you see it pop up in iTunes, hold shift (Windows) or option (Mac) and click restore. Now you should be able to restore your device.