I don't know if this will help, but I was having problems updating an iPhone 3GS. Someone else here on McRumours (thanks TriJetHero) posted the following steps to get rid of my issue:
Step 1: Download and the latest version of TinyUmbrella from
http://thefirmwareumbrella.blogspot.com
Step 2: Click on the "Advanced" tab and uncheck the option "Set Hosts to Cydia on Exit"
Step 3: Reboot your computer and try to restore again.
Along the way TinyUmbrella had told me to kill Skype and I also read I needed to disable my antivirus software during the update. After I performed the steps above, I also made sure Skype was not running and temporarily turned off Symantec Endpoint Protection. I was then able to upgrade my phone.