It may depend by the codec used for the decoding process...
Which codecs do you use? If you have a computer with +700MHz CPU, then you could use DivX codec for DivX and generic MPEG4 files (and maybe XviD for xvid files).
If your CPU is slower, maybe try using FFDShow and BSplayer version 0.86.501 (read BSPeter's Help2Help post to know where to download it).
If you search the forum you can find several posts regarding slow cpu configurations.
Another possibility is to change the overlay settings in BSplayer preferences
P.S. BSplayer is low in CPU and memory consumption (maybe the lowest player on the market)