how do you convert a video file to another format

[c=003BB2][i]i have a folder on my computer with just video files in, many files are 'matroska file' format and i would like to change the file to mpg or is it wmg? i can't remember....but just a more commonly played and used file format...do you get what i mean?

i use a windows 7 computer at the moment.[/i][/c]