Reset 8u2 on Arduino Mega 2560
I have confirmed that, on my UNO R2, if I bridge the two connections shown briefly, the 8U2 resets and enters DFU mode.  Instantly, the logs on my Linux system that it is connected to show that the ACM device (what would be COM22 for you) has been removed, and replaced with a generic USB device.  The dfu-programmer command can then communicate with it fine.

The bridging of the connection should be done:


while the Arduino is connected to the computer and powered up,
briefly, with a bit of wire, resistor leg, screwdriver, tweezers, etc - whatever you have to hand.


Removing power from the Arduino and re-connecting it returns it back to normal operation.


Incidentally, the UNO R2 has that block of 6 pins populated with a header - makes it easier...