What I did (I was so fortunate to have another PC, where I didn't upgrade DivX) was simply search (through [Start]) for the file divxdec.ax.
Then I renamed it (make sure when doing so there is no player active so that this .ax could be in use), e.g. to Xdivxdec.ax. Subsequently I placed the divxdec.ax file from my other computer in that same (system) directory.
You may use the same file as I used (as attached).
(You will first have to unzip it and after having placed it in the (system) directory where you found your newer version, you will have to rename it to divxdec.ax by removing the numbers in front of its name which indicate its version.
Mind you: by first renaming your existing file (instead of removing or overwriting it), you can later at any time "undo" your "downgrading" and activate your newer DivX-version again.
(P.S.: If I remember correctly, I first had to restart my computer before it worked)
See my post furtheron of Mon Sep 08, 2003 10:21 am (Attachment removed!)