The solution: There is an old NT utility called SubInAcl.exe that can be used to reset permissions in Windows XP and also in Vista.
Vista's User Account Control (UAC) must be disabled for these steps to work (Control Panel -> Turn User Account Control Off). You may enable UAC back on after these steps. And you should run this as admin.
[/B]Download subinacl.msi from Microsoft and install it.
Download a modified version of reset.cmd from here and save the file in 'C:\Program Files\Windows Resource Kits\Tools'.
for Vista 64-bit: to 'C:\Program Files (X86)\Windows Resource Kits\Tools'
Open Command Prompt (press the Windows Start button and type CMD) and type the following without quotes (hit Enter): 'cd c:\program files\windows resource kits ools' for Vista 64-bit: 'cd c:\program files (X86)\windows resource kits ools'
Now type 'reset.cmd' and hit Enter (without quotes). You will see the Command Line Tool running for some seconds. The issue should have been fixed and Quicktime should run again.