danick1991,

As I stated in my earlier post, if the loopback does not work then you have isolated the problem to the 16u2 side of the board. Until that works, I would not even worry about the atmega328.

At this point, it seems as if the 16u2 has been damaged and you should could consider getting a new board.

Those are the drivers. The UNO uses the Windows built in usbser.sys driver. All that is needed to use that driver is a .inf file to instruct Windows to use that driver with the UNO.

danick1991,

Remove the atmega328 and try the loopback test again.
Make sure you connect a jumper from the TX pin (Digital Pin 1) to the RX pin (Digital Pin 0).

If the loopback works, then there is something wrong with the atmega328.
If not, then there is something wrong with the 16u2.

Select Show verbose output during upload in Arduino preferences.

Post back with the upload output and maybe that will give us an idea of what might be happening.

Take a look at this issue: http://code.google.com/p/arduino/issues/detail?id=806

Edit your boards.txt file and change
From:
mega2560.upload.protocol=stk500v2

To:
 mega2560.upload.protocol=wiring

Just fyi

There is already an issue opened for this problem: http://code.google.com/p/arduino/issues/detail?id=200

The sequence can be anywhere in the hex file that gets uploaded.
If you have a byte array that contains the three or more bytes in a row, that will trigger it.

Look for any array that has 0x21 0x21 0x21 or the decimal equivalent 33 33 33

The following procedure will help you install the drivers for your Arduino board on Windows:

Go to Device Manager
Right click the unknown device in Device Manager \ Other Devices
Select Update Driver Software
Select Browse my computer for software
Click Browse button
Select the Drivers folder under the location of your Arduino software installation. (Do not select the FTDI folder inside the Drivers folder)
Click OK
Click Next

There are two firmwares loaded on the 8U2/16U2
DFU bootloader and USB Serial firmware

The COMBINED hex is meant to be loaded via the ICSP header and will program both the bootloader and USB Serial firmware.

The Arduino-usbserial-atmega16u2-Mega2560-Rev3.hex is only the USB Serial firmware and is meant to be loaded by the DFU bootloader when in DFU mode.

What version of Uno do you have?
What version of the Arduino IDE do you have?

Open Device Manager
Right Click the Device Manager \ Other Devices \ unknown device
Select Properties
Select Details tab
Choose Hardware Ids from the Property drop-down
What is the Hardware Ids that show in the Value section?

If you are using DFU mode to flash the 16u2, you need to use this firmware image: (Do not use the one with "COMBINED" in the name)

hardware\arduino\firmwares\arduino-usbserial\Arduino-usbserial-atmega16u2-Mega2560-Rev3.hex

The following procedure will help you install the drivers for your Arduino board:

Go to Device Manager
Right click the unknown device in Device Manager \ Other Devices
Select Update Driver Software
Select Browse my computer for software
Click Browse button
Select the Drivers folder under the location of your Arduino software installation. (Do not select the FTDI folder inside the Drivers folder)
Click OK
Click Next

This reminds me of 'Due'ke Nukem Forever, that took 15 years from announcement to release.

I would suggest moving the Arduino folder outside of "Program Files".

This is the convention that I follow and I have not had any problems:
C:\arduino\arduino-1.0
C:\arduino\arduino-1.0.1
...

The following procedure will help you install the drivers for your Arduino board:

Go to Device Manager
Right click the unknown device in Device Manager \ Other Devices
Select Update Driver Software
Select Browse my computer for software
Click Browse button
Select the Drivers folder under the location of your Arduino software installation
Click OK
Click Next