you're probably missing a codec. I usually install divx and it runs just about everything I need. www.divx.com. There may be other codecs that are required that divx don't cover though...
yeah, download <a href="http://www.afterdawn.com/software/video_software/codecs_and_filters/ffdshow.cfm">ffdshow</a>. it's got a whole bunch of codecs together. the more codecs you have, the better the quality of playback
if you're using ie, yannic, click on tools, internet options, and in the general tab click "delete cookies"
if that doesn't work, and you're using windows xp, go to my computer, c:, documents and settings, your account file, cookies, then select all the files and delete them. usually they all get deleted with the exception of index.dat
uhh, well, if you have both ace mega pack and ffdshow, maybe the problem is too many codecs that conflict with each other. trying uninstalling a pack or two
if that doesn't work, uninstall all of them and download divx and xvid individually like josh said
it depends on the file type that you're trying to play. for avis, there are a number of codecs out there like divx and xvid. same goes for files with extensions such as .mkv and .ogm. they need their own special codecs.