This article shows how to install the USB driver for the Arduino UNO in Windows 7 when it is plugged into the computer for the first time.
What you found from the Window device manager is that there is an Arduino Uno device
Double click the unknown Arduino Uno device, a property window pops up
Choose the 'Driver' tab, and select 'Update Driver...'
Go to the Arduino IDE (also known as the Arduino Software) directory, and select the 'drivers' direction, then click 'OK'
Do a final check to make sure the driver has been correctly installed.