Was this by any chance video from one of those pocket camcorders, like the Flip? They tend to use strange codecs for their video and audio and sometimes you get issues like this. They're designed just to shoot video and load it to YouTube -- not actually do serious stuff like create DVDs from it.
In any event, you may want to see if the file opens in Vegas, and then use Vegas to export it to a more traditional DVD-quality MPEG before you bring it into DVD Architect.
This is the challenge with downloading files from the internet. You have no idea how they were encoded. But you won't be able to use them to make a DVD until you find a product that can convert them to something more standard!
You can try Prism Video Converter. See if it will make it into a DVD quality MPEG2.
If not, you'll likely be able to convert it with Quicktime Pro from Apple. But it costs $29 (though well worth it, if you deal regularly with MP4s and MOVs).