Amateur night.
DX50 is the fourcc for DivX 5. Although, now that it's 5.1, you'd think they'd change it to DX51. Anyway, if you have the DivX 5 directshow decoder (if you downloaded divx 5), it should play in ANY directshow player. That means divx player, windows media player, penis player, whatever
There are no xvid binaries on
www.xvid.org, and the only build that comes with a fourcc changer is
Koepi's.
Now if you have the divx decoder and it isn't being decoded with it, my only guess is the codec fourcc is different, so if you do get that fourcc changer, make sure it says DX50 and DIVX, and if it dosen't that's probably the problem.