This post explains how you can resolve issues with iTunes not installing or updating when you get a Windows Installer package.
What is the issue with Windows Installer package
This issue can occur if the Windows Installer package has stale information about the state of iTunes on your system. This can occur normally due to the corruption of information about iTunes and there may be no direct cause.
Typically this issue will stop you installing or upgrading iTunes and will show the following message.
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.
Solving this Windows Installer package issue
To fully clear the install information about a windows package you can use the Windows installer repair tool. This is available from the Microsoft Website here.
Running this tool will let you remove all information about programs and their install history. You will want to use this several times and remove the details of all iTunes and related products.
Please note this will fully remove the details of these programs from your computer.
Once all the iTunes installer entries have been removed you should be able to then reinstall iTunes by downloading the installer from their website.