codecs

I bought a 360 today (yay!), and I have it linked up to Media Center in Vista. Now here's the deal. Some of my videos don't play because they need new codecs (Xvid/Divx). As far as I've seen, there's nothing from Microsoft for this. So does anyone know how I can install these on the 360 easily?

What the hell are they, and why do I need them on my Media Player so I can watch movies I downloaded? And where can I get some? I got this cheesy new movie I want to watch...