Let me explain it another way. If you install the arduino IDE and then install the proper USB driver that supports the Uno and/or Mega rev 3 boards you will have the required one serial USB driver installed on your PC. However if you then proceed to plug in 4 different Uno boards the PC will create a new serial driver for each new board, based on the "base driver" and assign each a new different COM port number. This is just the way it works and it does bother some people, and there is a way to make the PC reassign the COM port number to be all the same if you wish. I'll leave it to others as to how you can do that if you wish, as it probably matters what OS version you are using. But the fact remains that if you plug the board in and select the proper COM port number in the IDE it will work with the board.
Or is it the case that you can't get the IDE to talk to any one specific arduino board you plug into it?