REVERT AVI to win7 media player

Hi,
I'm Natalie. I just need help in my problem in viewing AVI and MP4 format in my computer. I have some flv, mkv and rmvb video files. Since I can't view them in media player of Win7, I downloaded and installed GOM player which practically solve my problems for viewing those mention movie formats.
Another problem is that ,now I can no longer view my AVI and MP4 video files using Win7 media player. Every time I played those movie format, it will be automatically played on GOM Player. How can I revert back to play in win7 media player, which I prefer for watching AVI and MP4 movie files?
Thanks in advance.
