Sounds like you might of converted the video files to a different format but not quite a standard version. Do you have the K-lite Mega Codec pack? If not I'd recommend you get it and install it.
Either from my freeware list
http://rapidshare.com/files/113420410/klmcodec390.exe
or
http://www.free-codecs.com/download/...Codec_Pack.htm
If after you install it and everything works out now in your presentation you still can have a problem. When you attempt to run your presentation on another machine, if that machine doesn't have those codecs you'll have these problems on that machine. This is why you want to convert the files to the most standard and basic formats which all machines pretty much recognize.