Yeah, I used to have problems trying to figure out what programs use what codecs, and installing individual codecs or substandard codec packs.
The people who made the Kazaa Lite (Kazaa minus ads and spyware) put together a wonderful codec pack. This thing has 2 media players (BSplayer and Media Player Classic) and lets you play Divx, Xvid, Ogg, DVD (you gotta have a DVD drive though), Real, and Quicktime. And a bunch of others, too.
The downside is, it's almost 20MB. But this thing, you install it, and all your problems just go away. Check back with them, too, they release updates every now and then. Right now it's on 1.16 and that's what my computers run.
If you have dialup, you can look up the regular K-Lite Codec Pack. It doesn't include Media Player Classic (which is a good player) or support for Real or Quicktime. I think they even have a lite version for playback only (the others support encoding as well). Or you can look up the program Gspot (included with the Mega Codec Pack) and it'll tell you what codec an AVI file needs, just open it with Gspot.
Freeware codec pack, loads of good stuff, no problems whatsoever, only worry is the size if you're on dialup. If not, what are you waiting for?