A random problem with the Quicktime 7.5 update

Even though a number of us have had no problem installing the QuickTime 7.5 update, some (see here and here) are confronted with serious problems.
Either their machine starts again in an endless loop, or it starts up, but refuses to launch Finder however all the other software functions correctly.
It is still too early to know the common link between those who have this problem.