This usually occurs if the “Temp” folder on your hard drive is full (this is the folder temporarily used by installers, including the QuickTime installer). If there is too much in this folder, the installer may not work properly. You should empty the “Temp” folder (if there are any old files in this folder which you need, move them to another folder) and then run the QuickTime installer again. You should close all other applications, including any anti-virus software utilities, when you run the installer. If you still encounter errors, you may wish to visit Apple’s QuickTime page for support help: http://www.apple.com/quicktime.