Drivers provide a way for software on your computer (i.e. the Arduino software) to talk to hardware you connect to your computer (the Arduino board). In the case of Arduino, the drivers work by providing a virtual serial port (or virtual COM port). The Arduino Uno and Mega 2560 use standard drivers (USB CDC) provided by the operating system to communicate with the ATmega8U2 on the board. Other Arduino boards use FTDI drivers to communicate with the FTDI chip on the board (or in the USB-serial convertor).The easiest way to check if the drivers for your board are installed correctly is by opening the Tools > Serial Port menu in the Arduino software with the Arduino board connected to your computer. Additional menu items should appear relative to when you open the menu without the Arduino connected to your computer. Note that it shouldn't matter what name the Arduino board's serial port gets assigned as long as that's the one you pick from the menu.On Windows 7 (particularly the 64-bit version), you might need to go into the Device Manager and update the drivers for the Uno or Mega 2560. Just right click on the device (the board should be connected to your computer), and point Windows at the appropriate .inf file again. The .inf is in the drivers/ directory of the Arduino software (not in the FTDI USB Drivers sub-directory of it).If you get this error when installing the Uno or Mega 2560 drivers on Windows XP: "The system cannot find the file specified", you might try this suggestion (about adding a "RunOnce" key to "HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion").On Linux, the Uno and Mega 2560 show up as devices of the form /dev/ttyACM0. These are not supported by the standard version of the RXTX library that the Arduino software uses for serial communication. The Arduino software download for Linux includes a version of the RXTX library patched to also search for these /dev/ttyACM* devices. There's also an Ubuntu package (for 11.04) which includes support for these devices. If, however, you're using the RXTX package from your distribution, you may need to symlink from /dev/ttyACM0 to /dev/ttyUSB0 (for example) so that the serial port appears in the Arduino software. Run: sudo usermod -a -G tty yourUserName sudo usermod -a -G dialout yourUserName Log off and log on again for the changes to take effect.