iPhone 3gs in Windows 7 files explorer

Hi All,

I've a Windows 7 64 bits up to date and an iPhone 3gs that cannot be seen in windows 7 files explorer when plugged with USB.

It appears correctly in iTunes and I can add music.

My friend has an iPhone 4s and on the same computer his iPhone appears in the windows files explorer.

I've looked at the driver and asked for "update driver" but I've been told that it's already up to date.

What can I do ?

Thanks,