First try this fixit:
If that doesn't work you may need the the Microsoft Installer Cleanup Utility
Unfortunately the Microsoft Installer Cleanup utility has been withdrawn by Microsoft because of compatibility issues with Microsoft Office so you will need to find a copy elsewhere – the installer is msicuu2.exe. You need version 3.
Google msicuu2.exe download
mydigitallife in the above search has a copy of version 3
Check for malware before installing it.
== uninstall with cleanup ==
Download a fresh copy of iTunes
Save the file on your PC.
Now use the following method to remove iTunes and its components:
*If you hit a problem with one of the uninstalls don't worry*, carry on with the deleting of files and folders as directed in the method.
Restart your PC.
Run the Microsoft Installer Cleanup Utility. (Start > All Programs > Windows Install Clean Up)
If you get permission denied error, Try Run as Administrator – an option if you right click on the program.
Remove any references you find to the programs you removed - strictly speaking you only need to worry about those programs where the uninstall failed.
If you don’t see an entry for one of the programs that did *not* uninstall, look out for entries that start All Users that are blank entries or have numeric entries that look like version numbers
restart your PC and try installing iTunes