You can find quite a few results if you simply search for "free video file player," but here are a couple for you.

http://www.getmiro.com/

http://www.videolan.org/vlc/

This site has lots to choose from:

http://www.freedownloadscenter.com/Search/video_player.html

You probably already know this, but to optimize your video experience, make sure your computer has plenty of RAM (at least 2GB, especially for Vista which eats up your memory) and having a nice video card in there is always nice too. NVIDEA GeForce video cards are great.