Not a stupid question at all. Apparently either the encoder I use (autogordianknot, set to use basic Xvid codecs) or my cut & splice freeware (virtualdub) does some tricky encoding so that unless you have ALL the most updated codecs, these clips are picky about what they play on.

Try VLC Media player (http://www.videolan.org/vlc/), which is an awesome freeware media player that will generally play just about anything. You can also try dling more codecs for you 'puter:

http://www.xpcodecpack.com/
http://www.xpcodecpack.com/ (the xvid for windows one)
http://www.soft32.com/download_102727.html
http://www.soft32.com/download_2592.html