Can't update my iphone 3GS IOS. Tried everything I know. Please help.

Hi,

I am hoping someone can help me update the IOS on my iphone. I have tried all the suggestions on apple support site. I currently only have 4.3.3 installed so I am not able to install some apps or update others.

Why do I receive this message when I try to update my iphone 3GS? "An error occurred when backing up this Iphone. {-37} Would you like to continue to update this iphone? Continuing will result in the loss of all contents on this Iphone." I know this error regards security but I uninstalled my 3rd party firewall program

Have the latest version of Itunes installed
Unplugged all devices from USB except mouse and keyboard.
Used USB in back of hard drive to plug in my iphone..
Temporarily disabled Antivirus program.
Added apple websites to trusted sites
Ran Diagnostics - Passed all steps
Checked hosts file - not blocking itunes from communicating with the
update server
Verified Ports 80 and 443 are not blocked

Can anyone help, please? What else can I try?

Thanks,

Gail