AVI is not a video format, rather it is a container format that can have a wide variety of audio and video tracks within it. If a user received a file and that person's computer did not have the codecs to translate the audio and video contained in the AVI file, they would not be able to view and/or listen to the file.
"If i create a WMV or AVI, can i saftely assume that it will play universaly (on computers outside of the United States)?"
Right now the only format you can be pretty much assured of compatibility is Flash, which is 1 reason you see flv files everywhere. DivX/Xvid are making progress, & have the advantage that most DVD players now days can handle them, plus the Xvid codec install is a pretty small download.
AVI & WMV are OK *if* you stick with the codecs that come with all versions of Windows, which doesn't give you a lot frankly, & of course leaves out MAC & *nix PCs.
An alternative I've used time to time with discs is AVC/h264, which can give good looking results plus small files -- then I just bundle VLC for playback. VLC uncompressed is too big for a download, but is self-contained, & there's a portable version for use on systems where writing to the registry is forbidden. Just include the folder & it works. And you can set it up for auto playback.