iphone shown as "unknown device" on device manager

hi ,

my iphone 3gs (vers 3.1.2) working with itunes normally but shown as "unknown device" on device manager and i can't get my photos on iphone from "my computer" like removeble device

there is some solutions which i tried but problem still running ,

1. i have last vers. of i tunes, i tried to reinstall but same ,
2. clicked "unknown device" then "update driver" and tried to update from this location (C:\Program Files\Common Files\Apple\Mobile Device Support\Drivers) but can't find driver
3. tried to install another computer i tunes then plugged same iphone but same resoult happened
4. when i plugged another iphone to my computer , it's working normal
5. i tried to plug another usb port but same
6. reinstalled my operating system (windows 7 ultimate) but same


as i said before , i'm using my device with itunes without any prblem (i can sync and i can backup vs..) but i don't understand why i can't see my device on device manager ,i need to get my photos folder on "my computer" , waiting your help..

thanks