There wasn't any option for enabling all MPEG4 codecs.
I managed to get the file to work in DirectShow by changing the FourCC to XVID.
However, after reading your post, I decided to open the "Configure Decoder" option in the start menu and now I am getting an error about opening the ax file for it. Is this a Windows Vista 64 bit error (file paths different or something?).
Ps. I've just (after replying) rerun the installation of Xvid. I see what you mean, I enabled the all options tickbox. The configuration option in the start menu still doesn't work, but I think that's because the xvid.ax file is stored in the syswow64 folder instead of perhaps the system32 folder. Would it just be a simple case of moving the file and reregistering it?