How did you record the videos of what they were playing?
You probably have MPC (media player classic) set up incorrectly. In Filters, you might want to try enabling/disabling the various built-in filters for AVI/MPG etc and see if they change performance. I have the internal AVI splitter enabled but internal MPEG-1 disabled on my machines.
Under Output, Overlay is the fastest choice for output. Also don't run more than one media player at a time - only one can hold the overlay part of your video card at any time, which gives better video playback performance. If you were doing this test with them side by side (possibly plus Winamp as a 3rd in the background with a video paused in that too), then one of them is gonna make a huge impact on the performance of the others due to this, since the others can't use the accelerated features.
If you have the Real/Quicktime Alternate codecs installed, then that's already the RealOne and Quicktime players you can throw away. Technically that should be enough to get WM9 to play these two types, but I don't know how WM9 builds its internal graph to play the file so it might not work anyway. It works with MPC fine though. Get rid of the DivX player too, MPC and WM9 can both play AVIs.
As for what I recommend, certainly not WM9 with it's phone-home spyware in it nor for its screen-space bloatedness. MPC is the best for videos when you know how to work all it's features. I've not seen audio/video resynchronisation for out of sync videos, video aspect ratio correcting/altering for distorted videos, custom codec priorities for testing out multiple codecs installed on your machine, audio channel remapping, to name a few features, in either WM9 or Winamp, yet MPC can do all this. MPC can do far more than WM9 can do. I leave Winamp to what it's best at doing, and originally made for doing, and that's audio only.
I have my machines set up to use Winamp for audio, and MPC for all videos (avi, ogm, mkv, qt, rm, mpg, DVDs, VCDs). (Or XMMS and xine under linux.)
Or, you can go and install
gentoo, and emerge and use the xine media player - it may be the ugliest of the lot, but you can't see that in fullscreen, and it has the same, if not more, capabilities MPC has :p
EDIT: I forgot, Alcohol 120% and MPC don't mix well either :p Alcohol 120% is a poor program too as far as it's CD emulation goes, Daemon Tools is much better if you need this stuff :p
