why won't my Iphone show up in itunes?

I have deleted and reinstalled itunes multiple times, disabled and enabled all of my usb drivers, and reset my phone; still no luck. My computer recognizes the phone because it charges it, but it just won't show up in itunes. It isn't a problem with the phone because I took it to the apple store and tried to sync a brand new phone and had no luck also...PLEASE HELP

I've got a 3GS and am using windows 7