I am using an Uno R3 board.
Somehow I seem to have corrupted the firmware in the 16U2 that does the USB connection to the computer. I am using Windows XP. (I like it and have rejected Windows 7 for any serious development work).
The symptoms. I was unable to program the M328P and kept on receiving a synchronization error. The Uno board was visible in the Device Manager as a COMM port.
I obtained an In Circuit Programmer.
I have programmed the 16U2 with the 'Arduino-COMBINED-dfu-usbserial-atmega16u2-Uno-Rev3.hex' firmware without any problems.
Now when I connect the Uno board to the computer via the USB cable, a USB device is found. However, a driver can not be located for the device. I have tried the drivers as supplied by Arduino in the Arduino\Drivers folder and the drivers supplied in the Old_Arduino_Drivers.zip file with no success.