Have you tried CodingBadly's loopback test?http://arduino.cc/forum/index.php/topic,73748.0.html
I just tried it, and it failed the test. The RX LED flashes when I send something, but the TX doesn't, so I get nothing back.(by serial monitor he did mean the thing that pops up when I press CTRL+SHIFT+M in the Programming environment, right?)
Yup, that is the one. Okay, try this. Download FLIP and install:http://www.atmel.com/tools/FLIP.aspxLeave the software closed after install and plug in the Uno and short the Reset and Gnd pins on the Atmega8u2/16u2:This should cause a new device to be found in Windows. Manually navigate the wizard to Program Files\ATMEL\FLIP 3.4.7\usb for the .INF file for your DFU. After the installation is complete, open FLIP and then post back here which driver installed (Atmega16u2 or Atmega8u2).