Any idea why QuickTime won't open certain files on remote site?

I've read your post three times and still don't understand what QuickTime (the Mac's built in media player) in your title has to do with Norton. Norton what? Where does that come from?

Get rid of Norton.

Norton Antivirus (made by Symantec) has a very long and illustrious reputation for mangling Mac OS X systems, sometimes to the point where a complete reinstall is necessary. Among other things, it installs kernel extensions which are known to cause kernel panics and system freezes; it contains known and documented bugs which can silently corrupt Adobe Photoshop and Adobe InDesign files, destroy a user's ability to authenticate as an administrator, and (on PPC systems) can cause Classic to stop functioning; and Symantec has on at least two occasions now released flawed .dat file updates which erroneously report certain critical Mac OS X files as "viruses." (Deleting these "viruses" causes damage to the system that in some cases renders it unbootable.)

Norton Removal Tool (Symantec Uninstaller):

http://www.symantec.com/business/support/index?page=content&id=TECH103489&locale =en_US

And now this, from 11 January 2012:

Lawsuit Claims Symantec "Scareware" Warns Of Fake Threats To Sell Upgrades

http://www.forbes.com/sites/andygreenberg/2012/01/11/lawsuit-claims-symantec-sca reware-warns-of-fake-threats-to-sell-upgrades/

There are many forms of ‘Malware’ that can affect a computer system, of which ‘a virus’ is but one type, ‘trojans’ another. Using the strict definition of a computer virus, no viruses that can attack OS X have so far been detected 'in the wild', i.e. in anything other than laboratory conditions. The same is not true of other forms of malware, such as Trojans. Whilst it is a fairly safe bet that your Mac has NOT been infected by a virus, it may have another security-related problem, but more likely a technical problem unrelated to any malware threat.

You may find this User Tip on Viruses, Trojan Detection and Removal, as well as general Internet Security and Privacy, useful:

https://discussions.apple.com/docs/DOC-2435

The User Tip (which you are welcome to print out and retain for future reference) seeks to offer guidance on the main security threats and how to avoid them.

More useful information can also be found here:

http://www.reedcorner.net/mmg/

Can you provide a link to some of the files Quicktime does not play?

Quicktime cannot play all kind of files natively, particularly Flash files.

Firstly: delightful portraits on your web site! 🙂

Can you please confirm what versions of OS X and Quicktime you are running?

I don't want to register at myss.com but I managed to view one or two of her presentations without a hitch using QT 7.

Try using QT 7.

Mac OS X 10.6 includes QuickTime versions 10.0 and 7.6.3. The QuickTime 7 player will only be present if a QuickTime Pro key was present at the time of installation, or if specified as part of a custom install, or individually downloaded:

http://support.apple.com/kb/dl923

Snow Leopard update 10.6.4 included an update to 7.6.6 (if installed). You can install it from the above link even though it says for 10.6.3. It's the same version of QuickTime Player 7.6.6.

(Only QuickTime Player 7.6.3 or 7.6.6 can be updated to "Pro".)

A Mac OS X v10.6, OS X Lion, and OS X Mountain Lion-compatible version of QuickTime Player 7 is available for use with older media or with AppleScript-based workflows. QuickTime Player 7 can be used to playback formats such as QTVR, interactive QuickTime Movies, and MIDI files. Also, it supports QuickTime 7 Pro registration codes for access to QuickTime Pro functionality.

How to install Quicktime Player 7 on Snow Leopard, Lion and Mountain Lion when it is not already present:

http://support.apple.com/kb/HT3678?viewlocale=en_US&locale=en_US

dphoeb wrote:

Thanks for the problem solving! Wonderful thought, although it didn't work.

I was only monitoring this thread in case something came from using ClamXav, so I don't have a lot of time to play with it, but decided to see what was going on for myself.

I'm almost certain the problem is with the web site. I tried to download Part 5 Session 1 and it was not able to find the file. I suggest you contact them and report that Parts 5 and 6 are not accessible.