All you need to do is "rip" the movie from your DVD to your computer. (ie. copy the raw file from the DVD to your hard drive) You may have already done this - it wasn't clear in your post.
After that is accomplished, try one of the below websites, which allow free conversion of file types for circumstances such as this one. (They all do almost the same thing, with slight differences.)
http://www.mediaconverter.org/index.php
http://media-convert.com/
http://online.movavi.com/
http://www.sr-ultimate.com/tag/free-...deo-converter/
http://www.cellsea.com/media/vindex.htm