Hi, I am witnessing very strange behavior around the same problem:
I have two Arduino Uno's. When I plug one into my computer, everything works fine, and Device Manager says it is Com4. When I plug the other one into the same USB port, Device Manager gives the topic error ("This device cannot start. (Code 10)") and says it is on Com5.
I tried replacing the Atmel ATMEGA328 chip on the "faulty" board with a new one off Ebay, but with the same error.
The other odd thing is that when I plug in the "good" board, the L LED (pin 13) is off. When I plug in the faulty board, the L LED is always on (when I hit the reset button, it flashes quickly a few times and then stays on). When I ground pin 13 it goes off, but comes back on immediately after I unground it.
So it seems there is something faulty on the board, but it's not the Atmel chip. By the way, the board was working fine for several months before this came up. I'm flumoxed, any ideas?
In case it helps others, I found a partial solution to the above problem. I still can't get my computer to talk to the "faulty" Arduino Uno via USB cable, but I can program it using my "good" Arduino Uno. First, wire them together as shown here: https://dlnmh9ip6v2uc.cloudfront.net/assets/3/b/d/8/1/523b53bb757b7f01268b456a.jpg / https://learn.sparkfun.com/tutorials/installing-an-arduino-bootloader. Then open the sketch you want to upload, go Tools / Programmer / Arduino as ISP, and then Upload. It uploaded the sketch to both boards, and the "faulty" board successfully ran the program (but still wouldn't communicate via USB).
It's going to be a pain to wire two Arduinos together each time I want to program the faulty one, but if it doesn't need to be programmed often, than this is better than bricking it.
By the way, before stumbling on the above solution, I tied reinstalling the bootloader on the "faulty" board, but that didn't fix not-communicating-via-USB problem.