Using Arduino nano - Arduino IDE serial port grayed out on windows 7

I am using a Arduino Nano 3 (with chipset ATMEL MEGE 382P AU 1437).

IDE:
When I launch the IDE I select the board > Arduino Nano w/ ATMega 328.
But the I am not able to select the Serial port since it is greyed out.

Serial Monitor:
If I try to launch the Serial Monitor, I get a exception "Serial port COM16 not found".

Cable:
I am certain the cable is working, because when I plug in the board the PWR (power) LED lights up and the LED with L on it blinks.
But I dont see the TX or RX lighting up.

Device Manager:
I tried opening up the Device Manager and setting up the COM port (as I had done for the Adruino UNO earlier on the same machine). But I dont have a Nano .inf file.
Updating the driver for the "Unknown Devices" with the Arduino/drivers/arduino.inf file did not help.

Any pointers will really help here.
I tried searching on Google and the Forums but I did not find any discussion around this.

On a separate note the Arduino UNO works fine on the same machine.