There are two different versions of the Arduino NANO, one uses the FTDI chip and the other utilizes the CH340G chip wich is identified by look on the bottom of the Arduino NANO. Windows 7 had some issues with the CH340G drivers and the following should help you get started if having issues.
CH340G converter Windows 7 driver download
Here you can download latest version of verified & working Windows 7 driver for Mini USB 2.0 to TTL 6Pin CH340G Converter sold as BTE13-009/USB.
Looks like the updated version from their site works now as is. Here is the updated CH341SER_MAC.ZIP (148KB) cached locally, but for other platforms, please checkout their website.