Over on The Green Button Media Center community website, I often see questions about codecs and members not being able to play this or that format in Media Center. I don't claim to be an expert on codecs but from my own experiences it is best not to install multiple codec packages as they can conflict with each other, more is not better in the case of codecs.
Allot of the questions ask how to play and manage file formats such as DIVX/XVID (AVI) and H.264 (MP4) in WMP11 and Media Center. The answer is pretty simple, uninstall any currently installed codec packs you may have installed such as the DIVX Pro Package. Strip your system back down and then install the FDDShow Tryouts directshow filter and the Haali Splitter. That's all you really need to ensure WMP11 will be able to add these formats in to it library and in turn Media Center will also be able to play these movies. Alternatively you could just install the K-Lite Full Codec pack which is what I use.
Some people recommend against using any find of codec package and just to use FDDShow Tryouts and the Haali Splitter, I have had no issues with the K-Lite Full Codec pack and I find it plays all of the movies I download from various sources on the Internet.
Another useful tool is GSPOT if your having issues playing a problematic AVI file for example you can use this utility to try and work out what codec that particular movie might need to be able to play it.
Also another good tip for the Linksys V2 extenders, if you are having issues playing back certain movies on the extender you might want to look at the Fourcc Changer utility. Because the DMA 2100 and 2200 have issues playing AVI files if the fourcc code in the file is set to DIVX or DX50.
Changing the fourcc code to XVID instead using the Fourcc Changer utility then allows the files to play fine.
More information on Chris Laniers blog here
Download Fourcc Changer here