cannot open any mp4 files on Quicktime but plays ok within iTunes.

Hi, I'm new to apple discussion forum, I have been searching for answer to my problems but couldn't find the resolution.

I have dozens of MP4 files which plays well on Quicktime on my other systems running XP-Pro. Recently I transferred my mp4 files on a portable drive and I wanted to open them up on a system which has 64-bit Vista. I can open all the files within iTunes without a problem (although it had to install files within iTunes first) but the same mp4 files won't run on Quicktime.

I wanted to be able to run the mp4 files on Quicktime without the need to open iTunes first, as I don't want to install these mp4 files in iTunes.

If any of these mp4 files are opened on Quicktime, I am getting an "Error -43: file not found..." message although the files are there. I don't think it's related to the 8 character limitation because I even tried to chunk-out the characters. I have tried many times to uninstall and reinstall both iTunes and Quicktime and also tried many times to run the standalone Quicktime setup by itself, but the same error-message remains.

I tried to run the mp4 file on RealPlayer as well as DivX but both program says it needed Quicktime to run the file, although Quicktime was installed it seemed like the programs cannot see quicktime, and yet when I run any of the mp4 files within iTunes they work fine.

Any reason why Quicktime won't open file and yet iTunes can open the same file?

I don't know of any but is there any known Quicktime compatibility problem with 64-bit Vista and if so what causes the conflict, or are there any workaround or patches? (can't find any related issues on Microsoft support)

Is there any way I can resolve this issue, and be able to open these mp4 files within Quicktime without getting error -43 message and without the need to install the mp4 files within iTunes first?

Any help from any good people out there are very much appreciated. Thanks very much in advance.