XviD was designed to be MPEG-4 spec compliant from the start, I believe. So you should have no problems decoding from the latest build.
Q: I'm running an i686, I have the latest tarball, MinGW, MSYS and YASM. "configure" detects YASM, "make" compiles everything including the ASM modules, but it won't finish the installation. What's going on?
This is a bug with YASM on this system type. It will NOT compile the ASM modules properly. Either wait for a newer version of YASM or get NASM, which does work on this system type. Get NASM
here.