Check your QuickTime MIME Settings preferences to make sure they are all enabled.
Quicktime>Quicktime Player Preferences>Advanced
Click on the
MIME Settings button.
In the next window that opens up make sure that
Streaming-Streaming movies is enabled (checked).
While you’re at it make sure all the boxes are enabled by clicking on the corresponding triangles so that they are pointed down.
This will show the “hidden” boxes especially the
Video box. This box will enable you to play .avi files.
Click the
OK button.
------–
Now click
Streaming
Your settings you should be (as starters):
Streaming Speed:
Automatic
Make sure
Enable Instant-On is enabled (checked).
------—
Now click on
Browser
Make sure:
Play movies automatically
Save movies in disk cache
Are both enabled.
Movie download Cache Size:
Slider set all the way to the left.
======================
Safari/QT MIME Plug-in Problems
Courtesy of user: andybal_uk - cheers!
If you've used the "installed plug-ins" command from Safari's Help menu & see no mention of QT, then No, of course that isn't normal.
You should see a section headed-
QuickTime Plug-in 7.x.x
The QuickTime Plugin allows you to view a wide variety of multimedia content in web pages. For more information, visit the QuickTime Web site. — from file “QuickTime Plugin.plugin”.
then a long list of mime types that the plugin handles.
So - assuming you don't see that, either the plugin is missing for some reason or there's a conflict that doesn't allow it to load.
Take a look in your Home/Library/Internet Plug-Ins folder - if you see any QT plugin or webplugin file there, remove them to the desktop or trash.
Now look in HD/Library/Internet Plug-Ins/ amongst other files, this should contain QuickTime Plugin.plugin
IF you found a QT plugin in the Home account plugins folder & have removed it, you should quit any browsers & re-open them - hopefully you will now have QT functioning.
IF you have no file 'QuickTime Plugin.plugin' then it seems something went wrong with the installation or a recent update.
=====================
.rar files can only be played using RealPlayer. Most websites provide an option to select this player if that is the
only type of file available on their site.
=====================
The players that you have downloaded, make sure they are specifically for your model mac-
Intel. Some websites will show

Hopefully, you are selecting the
Universal symbols for your downloads. Some websites may state "Intel package."
Also, it is highly recommended that you repair permissions after installations. See Knowledge Base Article:
http://docs.info.apple.com/article.html?artnum=25751 About Disk Utility's Repair Disk Permissions feature
=========================
As a side note: I have never had any problems playing .avi files using QuickTime. However, since I have an extensive .avi collection as a carry over from my OS 8.x-OS 9.x years I elected to install & make default VLC when I switched over to Tiger & QT
Pro. This way, VLC plays all of my .avi files beautifully.
I am new to the CAT Systems. Today I am celebrating my "first anniversery" (new computer & OS). So I guess the honeymoon is officially over!