A quick Google search turned up this article: http://www.howtogeek.com/111859/how-...ultiple-files/. His method for opening a Command Prompt window (one of the 4 methods, and probably the best for your need) requires a little clarification. In Windows 7, you put the mouse pointer over the name of a folder, hold down Shift, and right-click to get a menu with the Command Window option. Of course, you can always click the Start button, choose Run, and type cmd. From there you'll have to navigate to your BW folder, though.
I don't actually use Windows Explorer, but Directory Opus, which has a built-in multiple-file renaming facility (not terribly intuitive to use, unfortunately). There must be free or cheap utilities to do this too.