Well, it depends on what you're playing. DO NOT INSTALL DIVX 5.0. It's a pain in the ass.

If you have both Xvid and Divx 5.0 installed on your comp. They will both try to render the .avi files. If your .avi is either xvid or divx.
Right now I only have Xvid and ffdshow installed. My ffdshow decodes divx 3.11 divx 4. While my Xvid decodes Xvid and Divx 5.0.

I also have other stuff for MKV play back and OGMs.

This keeps things much cleaner.