C:\Documents and Settings\All Users\Application Data\Apple Computer\Installer Cache\QuickTime 7.73.80.64\QuickTime msi - you might be able to double click on it and have it reinstall the current version of QT that is on the computer, then do the update
C:\WINDOWS\Installer - has three msi files pertaining to QT
Apple Software Update Installer msi
Apple Application Support Installer msi
QuickTime Installer msi
If they are missing or you're not having any luck using them you'll need to download and install a new version of QuickTime. Where you cannot I would recommend downloading and installing Windows Installer Cleanup Utility. Run it then delete any old QuickTime installer that may show up in there.
http://majorgeeks.com/download.php?det=4459