Welcome back
beholder4096 
About DivX able to decode XviD files, yes, it's normal if you enabled (it's enabled by default) the option to play also other MPEG-4 videos as shown below
(check the option higlighted to enable XviD decoding)
The same applies to the contrary, XviD is able to decode MPEG-4 encoded with other codecs videos
Good to know you was able to increase the skeek frames speed, although I noticed better performances by using FFDShow or XviD decoders instead of DivX ones (well, v5.x was quite good)
Hope you'll get better seeking speed with WMV files!
See you soon
Tizio