I would suggest checking your drivers for your USB ports are installed. Check the USB port works with other devices (hard drive, pen drive, ipod, etc) Check your USB Cable is ok, try with another one. Check any Samsung Galaxy services are running on your machine. I know that if you don't have Apple's service running, Windows won't detect your iphone as a camera device in My Computer, Samsung could be similar???
Can you plug your Galaxy into another machine, does it detect it?