DivX videos aren’t something you need to go looking for. Many videos (like some on YouTube, certain blogs, movie rips, etc.) have been made using DivX codecs, and if you also have the DivX codecs on your computer, they will play back with the best clarity of sound and video. Once you’ve installed it, you’re done. It’s just a tool to help you get the most out of certain media that you come across.
(P.S. – Real Player is a disaster, trying to update itself all the time, constantly trying to access the internet while playing files without asking you, and hogging system resources worse than anything I’ve ever seen. If you have it, uninstall it.
Windows Media Player doesn’t play many formats, so is limited in what it can do for you.
iTunes is accepted as a terrific music player and library organizer, but is also limited in what it is good for.
If you were looking for a single multi-format player, as many others here would suggest, go to download.cnet.com and get VLC Player (search for it). It plays any format of anything that exists, audio and video.)