"There is a problem with this Windows Installer package. A program required for this install to complete could not be run. Contact your support personnel or package vendor."
Seems one of the iTunes components already installed is damaged. A quick remedy to this is to go to the Control Panel and Uninstall any Apple software - if you get any errors regarding a damaged or missing installer right click the item in the list and say repair. Once completed you should be able to remove it and install iTunes 10.5