Anyway, set up media server on the computer, then pick the files to share out with the server (UI for that is a little weird I found, but some googling makes it happen). Music, Photos, Videos all good. Then from the options in xbox (press the big round button in the middle of the controller) you should find "media player" or something like that, browse to files, play… done.
One thing to note is that on the XBox 360 there are two ways to stream stuff from your computer.
One is with the video/picture/music players built into the XBox. Works fine for music, pictures and most video. Using that method some video files won't work because the XBox doesn't support them natively.
The second method is to use the Windows Media Center on the XBox. Using that method you can watch a greater variety of video types because it's the actual PC playing the video file and the XBox is just 'displaying' it. Setting up the Windows Media Center is easy. Just start Windows Media Center on your XBox and write down the code. Go to your PC, start Windows Media Center, click 'add Extender' and enter the code.
More detailed instructions (for Windows 7):