Hmm, I remember xvid on 98 was sort of a cludgy pain but I thought I got it working.

I can’t remember if that was before or after retiring my k6-2 for a Duron 700

I can remember watching divx clips on my Biostar MB8500 VX chipset board with an Evergreen (Winchip) 200 MHz CPU with Win98 SE in the spring of 2001. But I have no idea where to get a codec that would support that now.
Something like this maybe? http://www.divx-digest.com/software/divxcodec.html

All the assembly functions in xvid are optionally used if certain CPU features are detected. In other words all builds should be capable of running on machines without MMX... assuming they've been compiled by older compilers that don't assume such features will be present and use them when processing the C files. They would be too slow to be of any use on anything without at least MMX though.

I've used XviD VFW codec on Pentium MMX before, under Win98 (and NT 3.51 too BTW), so I know the older versions worked at least. FWIW, the XVIDVFW.DLL I have is dated June 2009