have you tried checking for driver faults? Right Click My Computer, Properties, Hardware, Device Manager.... mines under image devices.... Apple iPhone. Right click that, properties, driver, update driver.

If that doesnt do any difference, do all that again, but this time uninstall the driver.

The next time you plug the iphone into the computer the PC will auto source the driver and reinstall it.

You might have to restart inbetween the uninstall and reinstall... not sure.