Since QuickTime is the primary media player for Macs, most 3rd party codecs install for use w/ QuickTime on Mac. Most 3rd party codecs for Windows will not work within QuickTime for Windows, so if you want to VIEW an XviD AVI file, you'll need to use a different media player. If you want to CONVERT an XviD AVI file to iPod or other QuickTime format, you'll need to use a different converter or at least some converter to first convert it to something QuickTime for Windows understands.
It's POSSIBLE, though not very likely, that renaming an XviD or DivX or other MPEG-4 based file to .mp4 might allow QuickTime for Windows to read it as a generic MPEG-4 file.
To see what QuickTime for Windows DOES handle standard, see
http://www.apple.com/quicktime/player/specs.html
