Thank you for your help. I couldn't figure out how to use Toast to open the files, but Parallels did the trick...sort of. Windows Media player played back only the sound track, no video. I copied the files to the Mac desktop, from where VLC could play them back. For some reason, the avi files caused Quicktime to crash.
The mutual incompatibility of computer media is frustrating. Windows-created files won't work on a Mac, VLC opens a file that causes Quicktime to crash, Windows Media Player will play sound but no video, there are a gazillion formats.... I should think most consumers are interested in untroubled access to media files, not in getting bogged down in geeky compatibility problems.