If you're going to play DivX/XviD on the 360, the video must be encoded with one of the builtin profiles - such as Home Theatre or High Definition.
If any of the .AVI/DivX files are created with the Unconstrained profile, uses Quarter Pixel (QPEL), or Motion Estimation (ME) the file will likely never play on the 360, but would otherwise work fine on a desktop/laptop.
It is also going to matter what audio stream is in the .AVI as well. I'm not 100% sure about the 360's list of supported audio codecs, but you can safely assume if the audio stream is a .mp3, the .avi will play.