How do I play H264/XVid rips using Windows Media Center?
I'd vote for the CCCP codec pack. But K-Lite is good. Make sure to let the installation register the codecs with Media Player/Center and you should be fine.
I just upgraded my divx to version 7. It supports H264 and Media Center is able to use the codec natively:
You need to download a Codec pack like this one
A google search for Codec Pack will provide you with more options.