Let me tell you, stay clear of codec packs. Codec packs are nothing but trouble.
Instead, here's an excellent
media setup guide.If you don't want to go through the trouble collecting each codec, I have already done it, so I might as well share it. Media setup guide codecs
rarred. With this, you know exactly what you have installed and all codecs works and are compatible with each other.
I recommend you to install DivX, Xvid and ffdshow codecs but configure ffdshow not to decode divx 4, 5 and xvid. Let the other two handle those.
The rar contains WMP9 only for WinXP, so get it from MS download site for other windozes. (No, you don't have to use it, just install it)