Connecting iPhone to PC Windows 7

I have iPhone 3G and Windows 7 PC. When I connect my iPhone to my computer, it is not recognizing it as an iPhone instead it shows up like a flash drive. In iTunes the phone is not showing up under "Devices". I tried to disable other programs that might conflict with iTunes and installing iPhone as Apple Support suggested, but that didn't help. Any suggestions how to solve this? My friend mentioned that I need to download some kind of driver for my computer. Where would I be able to download it? And what exactly do I need? I also tried to connect it to 2 other computers: on one more Windows 7 Toshiba it didn't work either on another Sony PC Windows 7 it worked... Please let me know if you've had this issue or you know how to solve it. Thanks!