Arduino Uno Device showing as unknown device in device manager

My arduino uno r3 is not getting recognised by the computer in the device manager. Because of this it shows unknown device in the device manager and i am unable to install the drivers.

I have also tried using the latest version of arduino ide. I am using windows7.

Please help me

What happens when you try to install the driver from the folder "C:\Program Files (x86)\Arduino\drivers" ?
Which properties are displayed with an unknown device?

Look here for help:
http://forum.arduino.cc/index.php/topic,178994.0.html

There are some arduino uno rev 1 with hardware connection problems. The symptoms are "unknown device" or a flaky connection. This is the "try another usb cable" type of problem. Some short cables work, longer ones tend to not work. The problem is hardware ...

Near the usb connector there are 2 resistors. They should be 27ohm or 50ohm. But they are 180 ohm. Exchange them with 50 ohm or solder some 100ohm smd on top of the original ones, and the problems are gone!

Good luck.