did you try a BIOS upgrade? also, try upping to the shared video memory to the max setting (i think it is 32 MB). i have the Abit NV7m and i have had no problems with DVD/Divx/MP3 playback. then again, i am running with two memory sticks, utilizing the full potential of the dual channel DDR nforce chipset. if you want dual channel, you will have to get another stick of memory and put it in the correct slot for your board. i have heard lots of bad things about the MSI nforce board and i have had nothing but bad experiences with them, so i don't know--it could be more insidious than memory.
is Ultra ATA mode on for your drives? maybe they are still in PIO mode under the IDE controller settings. that sometimes compromises playback. good luck!