finally managed it - the reason i couldn't rename it from m4v to mp4 was because first I had to turn off "Hide extensions for known filetypes" in Windows - once i did this, the extension was shown and i could change it ;-)
iTunes videos are most often M4V's and are DRM protected file types (that doesn't mean all M4V's are protected). There is no free/easy way to convert them to AVI /MP4/WMV - but I posted a link to a video that will show you how to do it anyway.