When I want to change the format of one of my video files, I always use Total Video Converter. At first, I get the trial version, which last for 15 days, but I found it so useful that I purchase the full version. The Standard version is not expensive, and it provides you with all the necessary tools to convert your video files.
It supports several video formats such as 3GP, MP4, DivX, MOV, FLV, etc, and this allows you to reproduce all your videos on your PC and your mobile and console devices. However, this software does not only change the format of your files; it can also burn your CDs or DVDs with the converted files, and can show photos as a slideshow. Nevertheless, the one of the strongest points of this program is that it also allows you to lightly retouch your videos. It offers you the possibility of change the size (to the extent possible), the audio codec and even the bitrate.
It can be set in 10 languages, so there is almost no possibility of that you do not know any of the languages to use the software. In addition, the interface is so intuitive that everyone will be able to use the software.