Appologies for continued hijacking
Yes, that should be all you have to do. Simply decompress the codecs you like into the appropriate subfolder (I believe its....codecs), and you should be able to play nearly any file. I don't remember the last file I threw at it that it couldn't play. It couldn't play some of the odd ones I've converted, but that's just because I made them poorly.
So yeah, all those formats are playable, as well as VOB files, so I play all my DVDs with it too, since PowerDVD can be a hog. Sadly, I haven't figured out how to get mplayer to be my program of choice for online Quicktime files, but it works for streaming those sometimes. It's just those ones that are embedded in the page that won't do it.
Anyway, welcome! I hope it works well for you too.
*edit*
Oh, and the place I go for the quick and dirty Windows installer is
here. Just get the "WinMPlayer Installer - The Movie Player for Win32 1.0pre5 (3.77MB)". I don't think you should need that Real Media 9 codec pack.