From Birnam Designs Wiki

Quicktime is installed in a way that it only functions under the account in which it was create. Many published fixes don't work for Vista. The answer is to use SubInAcl

SubInAcl.exe is a utility from NT days used to reset permissions. Download it here.

Download the reset.cmd file from here and save it. Save it in the same directory as the main SubInAcl file is -- C:\Program Files\Windows Resource Kits\Tools

Then open a command prompt as administrator and change to the directory in which you saved the file, and run the "reset.cmd" command.

This might take a while to run, but Quicktime should be fixed afterward.