Download and burn a live ubuntu CD/DVD, burn it, boot from it, go to a terminal, type dmesg. Plug in the Uno and note what changes.It will establish, without another physical PC, and without reinstalling your existing one, whether the Uno is seen as expected.
$ dmesg | grep tty
Are you trying these tests with nothing else connected to the board?What was the last circuit you had connected to the board?
Since Windows does not see the board, have you tried a different USB cable? Or tried using the cable on a different USB device to confirm it works?The cable is the weakest link in the lot. Granted, it is a gen-u-wine Arduino cable but it still could be defective.
Where did you get the defective one from?