ARDUINO UNO Won't connect to PC, tried everything!

I have had my arduino uno for about 3 months, it was working great untill it stoped being recognized by my windows 7 Toshiba. I tried, everything, re downloading the IDE and drivers, trying different USB Cables, different USB ports, different computers and various operating systems, and it wasn't recognized by any of the computers. When I plug it in it is not recognized in the device manager.

I always get the same message : serial port com 5 not found, have you selected the right one from tools > serial port.

But when I do that the Serial Port menu is grayed out.

This is the same for all usb cables, all usb ports and all computers.

So finally I decided i must have burnt out my Atmega 328, so I ordered a new one with the bootloader on it and it still dosen't work. I checked for loose and shorted wires on the board. I have tried uploading with nothing connected to it. The green led goes on when I plug it in to the computer and the led on pin 13 blinks once when I hit the reset button. I am so perplexed about what is broken on it.

Also nothing gets hot on it when I plug it in. I have also read through dozens of trouble shooting pages with no luck.

If anybody as any suggestions of what to do, or what you think I burnt out, i would love to hear them.

Thank you so much for the help.