How to play Nikon d5100 video on my computer?

It seems it's a quick time player file, but quick time plays it very badly. Any other alternatives?
or any good program that can convert my videos without changing the quality?

My video playing "swiss army knife" of choice is VLC, and it does pretty good with the QuickTime movies my Canon camera takes. It can be downloaded at http://www.videolan.org/vlc/.It also plays many other media formats in great quality as well (but I wouldn't recommend it for DVD's).

VLC is free and open-source, and it also won't be hobbled with ads or nags like some other programs.

If you are recording in Full HD video, it may be that your computers CPU is not fast enough to process all that data, nor do you have enough RAW to act as a buffer

Every time we start using cameras with higher and higher resolution, we need to upgrade our computers used to process the image files they produce. If your computer is older than two or three years old, it is time to upgrade

VLC media player is generally acknowledged to be the most versatile media player, and is famous for playing 99% of video file format regardless of the codecs installed on your PC