If you don't want to install any program to convert a singlefile, you only have to access Zamzar.
It supports the most known video formats (AVI, 3GP, MP4, etc.), image formats (BMP, JPG, PNG, etc.), sound formats (MP3, OGG, WAV, WMA, etc.), documents (DOC, XLS, PDF, etc.) and compression (ZIP, RAR, 7Z, etc.).
Finally, it also download videos from streaming services. You only have to copy and paste the URL and it will download and convert the video.
In short, a very useful online application that will allow you to enjoy your files on any device. The easy way to convert files online.