My installation of Media Center 2005 couldn't view Quicktime .mov files or decode any mpeg4 content in the "My Videos" area. This creates an interesting problem -- how is one to view all these lovely trailers on their MCE boxes? These are the steps I had to take to get these problems solved, it's probably worth mentioning that this can vary based on what mpeg 2/4 decoder software you have installed (I use WinDVD), and you should probably have QuickTime installed as well if you intend on decoding that content.
1. Let's tweak the registry to make sure Windows knows that this type is video. Open regedit, navigate to HKEY_CASSES_ROOT/.mov, if this does not exist create it. Once in the root of this add a string type called PerceivedType, set the value of this to 'video' without the quotes obviously.
2. The final step is that you will need an mpeg4 directshow splitter, luckily gabest wrote one. If you don't know who that is you've never used Media Player Classic, get out from under your rock and try it. Anyway the MP4 splitter is located here so download that, if you don't have 7zip to decompress it you can find that here as well. Once you decompress it, install it using "regsvr32 mp4splitter.ax" in a command prompt in that folder.
Now when you navigate to your videos in Media Center you can view your quicktime and mp4 files, which are much better containers than AVI but that's a discussion for another day. I didn't want to make a long blog about this with screenshots and such but I can update this if necessary for anyone who has trouble following some of these steps. Your mileage may vary, this worked for me on MCE2005.
Some sample screenshots from great HD 720P trailers on apple's site
