When I plug my new UNO R3 board into a PC's USB port it powers on but no device is detected by the operating system. I've tried on three different PCs, Windows 7, 8 and 8.1 with multiple different cables. My guess is that the board is defective but would appreciate any pointers you can give.
It shouldn't be detected by Windows. You must install the software for your operating system here. Scroll down to Arduino 1.0.5 and click on windows. after installing, open up the Arduino IDE (it should be labeled "Arduino" on the desktop or start menu/screen). Plug in your board, and go to Tools> Serial Port. Select COM3. If it is grayed out, it has been auto-selected for you. The reason it won't work without the software is that the UNO and other Arduino boards don't have "true" USB. They basically emulate a serial port over an usb connection.
I couldn't bypass com1 port in arduino 1.0.6. So I wired the arduino to a sparkfun on the anolog in: A0,A1, A2, A3, A4, and A5 via a microusb cable to the computer. Lay the sparkfun on it's face so the back is upward. Put a wire in the left pin and connect it to A5. The next to A4 and so on. Try the blink test. It prompted me to change my port because com1 was not available. So I changed to com3 and tried again. Now disconnect the sparfun and connect to the arduino directly via the printer port. Try the blink test. It prompted me to change my port because com3 was now unavailable so I chose com4. Try the blink test again. It should work.