On FastStone's site, they list this among recent improvements:
Version 2.4 (Mar 15, 2006)
* Now the built-in screen capture supports multi-monitor system
* Video files ( AVI, MPG, MPEG, WMV, MOV) can be displayed as thumbnails in the browser. Double clicking on a video file will open it in the default media player (Screenshot)
So maybe that's where you got the codec!
Thanks for the prompt to update my Faststone - off to download it now...