I'll admit it, I have some DivX files on my computer. Anyways, I use WMP11 to view them, courtesy of the DivX codec. This codec is external to WMP11, and comes with its own little nifty tray icon (quite annoying if you ask me).
So I was watching WMP11 generate thumbnails for the movies, and it was taking quite a while. Further inspection revealed the DivX tray icon flashing in and out of existence every few seconds. It seems that WMP11 starts and stops the codec for every movie that it processes. Pretty dumb.