Does Windows-7 already have something that does this? Or is there a free download that does it?
Somewhat of an duplicate of: Can I manually change an .mp4 to .m4v by changing the extension in the Finder (mac) or explorer (win)?
MP4 and M4V are essentially the exact same thing, M4V is usually used to indicate video only files but some sofware/companies are using it for standard MP4 files with video AND audio (lots of Apple software f.e.). It's just a convention to indicate the type of content. An audio only MP4 f.e. would have the extension .m4a. You can just rename the extension from .m4v to .mp4.