Windows 7 now has built-in support for DivX, XviD, H.264 out of the box! – so you no longer need to install codec packs etc, when playing back your favourite video (AVI) files
However even if you do Windows 7 won’t use them and use its own DirectShow Filters.
I’ve found this annoying as playing back some videos there appears to be a slight noticable lag with the audio and picture.
This handy tool will fix that and let you choose which codec to use for what format. i.e. use DivX Decoder to play DivX files, xviD to play xviD