arduino uno pc connection problem

Hi, Ive bought a new Arduino Uno R3 and an Arduino Mega 2560 few days ago.Ive connected and installed the driver successfully on my Windows 7 x86 PC for the Arduino Mega.But the prob is with the Uno.When I connect it to the PC USB port,no new device found message nor sound pops up.Ive searched on Device manager and scanned for hardware changes but it cant find the UNO device.Although the power LED(green) lights and the pin13 led(orange) blinks.I hopped for at least an "UNKNOWN DEVICE" showing as like my arduino mega with the same USB cable and port, but it doesnt. it seems very strange. can anyone suggest whether its a hardware problem or anytghing else? plz help