USB to COM cables
The cable works just fine under Windows 7, once you have the correct driver installed. The procedure you should follow is to first plug the USB end into your PC without connecting a scanner. If you are not asked for a driver, it could be because one is already installed.
When the cable is connected, the driver installed, and things are working correctly, you will see the cable show up as a COM port under the USB section in Device Manager. If you do not have the (correct) driver installed, that is also where you can launch to install/update to the correct driver. All you need is the FTDI driver for W7. It is used for many different devices and that's one reason you might already have it installed.
Also; you'll see the COM port number assigned to the cable from Device Manager. You'll typically need that to set the COM port in the software you are using.