Help w/ .avi playback PLEASE!

Question

OK I am having a strange issue w/ avi playback on my MacBook. They aren't playing, i'm just getting the broken QT icon instead. Here's some info:
• I noticed this directly after loading a movie onto a webpage I publish using RapidWeaver, which I then put in my web>sites folder through iDisk so I can access it using www.randombob.net .
• I loaded the movie (my first!) and then went to go online and check that it would play correctly. I got the broken QT icon.
• Tried it on my g/f's MacBook, identical to mine, and it loaded and played!
• Went back to mine, opened iDisk back up, navigated to the index.html file with the link to the movie, clicked, and it plays.
• Went out to the worldwide web to see if I could play other .avi files. I _COULD NOT_.
• Tried the same other .avi files on other's computers, _they worked_.
• Went back and tried going through iDisk again to the page just before the movie, clicked the link, and sure enough, it plays through iDisk again.
• Tried to flush cache, reset safari, repair permissions, restart computer, try another account, and no matter what.... my MacBook's browsers WILL NOT play avi's from the web, HOWEVER it will play them from my HD locally, and through *THE SAME HTML FILES* if you navigate there from idisk first.
Anyone have a clue what's the issue, and how to fix it?

Answer 1

OK MORE NEW information:
It only seems to affect avi files that originate on the web. What I mean is, I can't play my own AVI file off of my own website if I navigate there from the worldwide web address (www.randombob.net), but if I navigate to the index.html file in my iDisk that contains the direct link to the page the movie is on, it will load and play it in safari with no problems.
Furthermore, if I transfer a movie that I d/l'd that won't play to another mac and it does play, then renaming & recopying it back to MINE will not yield any benefit; it still won't play.
However, not all new AVI's are subject to bad behavior. if I take a new movie with my camera and load it onto the computer (via card reader onto the desktop), it works fine. These are the same files, from the same camera, that I first noticed the issue with.
SO, it appears that for whatever reason, avi files that originate on the web are not being treated properly, as if some header info or somthing is just plain being ignored. What's more, it has to be something that's tagged with internet avi's, because even if I d/l the file from the web on another computer first then transfer it to mine, it still acts the same way, showing no format in the movie inspector, and showing an all-white screen in QT.
So something with reading/resolving internet files? Anyone with suggestions short of "reinstall OS?" I don't want to have to go through that hassle for gawdammed avi playback.