the clips from the 'net have probably been encoded in Mpeg1 or Mpeg4 or using some other codec that compresses them a lot.
The software you're using is designed for DV-AVI, uncompressed video from a camera, so although it will support some of the clips you've downloaded, it won't support all of them, and these clips won't give you an acurate idea of the quality you'll get when using proper DV-AVI in your projects.
You could try and borrow someone elses camera and capture some footage, or I think some magazines have uncompressed sample footage from cameras to play about with if you don't have a camera of your own.
If you really want to use the stuff from the 'net, you'll probably have to convert it into a format that the software can use (DV-AVI).
There's plenty of software for doing this, like VirtualDub, Stoik video converter etc.
Panasonic SD9, Panasonic NVGS-75, Canon MV600i