Gosh it took me a half hour to log back in here .. I sometimes think they differentiate between the iTunes password and the Apple password .. so irritating.
Okay .. there is something I just found out that is important for me so obviously I am not an expert on this but now I am sure it is a codec that must be your problem and the reason why it is happening on your computer as opposed to (what did you say you were successful at using it on .. an external hard drive) your external hard drive is that the hard drive probably does not have other codecs installed. I have a million codecs (*.dll, *.ax) and if you do a search for those file types in your QuickTime folder you will see quite a few of them. I think though also there are some under Sys32 .. you know the Windows folder.
okay so now
http://www.apple.com/quicktime/resources/components.html?os=Windows&ctype=696d64 63&csubtype=574d5633
that link has some outside codecs that install on QuickTime .... BUT the thing that may be even more important is that if you are like me perhaps I did not purchase the
http://support.apple.com/kb/DL837
QuickTime Pro 7 registration code
I have tried to purchase as many codes or codecs that I could to make my QuickTime Pro work better .. and there has been no message for me to purchase a registration code after the one that I already have. But also I purchased the MPEG 2 codec and I am sort of lost on that one because I have to find out if that has an upgrade. The thing is .. is the version of QuickTime Player that I just installed SEPARATE from iTunes .. usually I upgrade them both at the same time but I was trying to see if there were some codecs that I could purchase separately for my QuickTime Pro and I noticed on the Apple QuickTime site that their version was higher than the one last installed with iTunes. So now I installed the separate QuickTime Pro lates version (I thought .. but when does that ever apply for longer than 24 hours ... geez I get tired of all of this upgrading!) and it says it is Version 7.7 (1680.34) ... which is annoying when you now ask a question about why there is no codec in your QuickTime Pro to be allowing it to open an *.avi file and it sends you to the link of the iTunes download (with QuickTime of course which is about 90 MBs .. at least ) and all the upgrade version says is 7.7!
This is all too frustrating I know. You have to really make sure that your iTunes program though is NOT associating with the *.mov file type. You have to see what program is associated with the *.mov and the *.qt file types. Then also like me check to see if you need to buy this QuickTime Pro 7 registration code. Then also do a scan of your *.dll and *.ax codecs and see if you have one that is interfering with the QuickTime Pro playing *.mov files.
Do you have another media player program besides iTunes and QuickTime? Like for example Windows Media Player? I have plenty and have been trying to get enough codecs so that I can just use all of the programs with all of the file types. (good luck)
well anyway hope I did not confuse you - If you find out anything about the QuickTime Pro 7 Registration code versus the 6 (I bought my QuickTime Pro way before even 6 came in) then let me know.