"not a valid short file name" and "invalid character" install errors
Symptoms
While upgrading or uninstalling a Windows software application (such as iTunes for Windows or QuickTime for Windows), you may receive one of the following error messages:
file name is not a valid short file name
The folder path ' folder name' contains an invalid character.
... where file name could be the name of any file, and folder name could be the name of any folder.
In the case of software that uses advertised shortcuts (such as recent versions of iTunes for Windows or Safari for Windows), the messages may also appear when attempting to launch software that has already been installed.
Possible Resolutions
Resolution 1
The error messages are usually caused by damage to the installation database for the application. Clearing the installation database for the application by using the Windows Installer CleanUp utility can usually get you past the error message.
(1) Download the Windows Installer CleanUp utility installer file (msicuu2.exe) from the following Major Geeks page (use one of the links under the 
thingy on the Major Geeks page):
http://majorgeeks.com/download.php?det=4459
(2) Doubleclick the msicuu2.exe file and follow the prompts to install the Windows Installer CleanUp utility. (If you're on a Windows Vista or Windows 7 system and you get a Code 800A0046 error message when doubleclicking the msicuu2.exe file, try instead right-clicking on the msicuu2.exe file and selecting "Run as administrator".)
(3) In your Start menu click All Programs and then click Windows Install Clean Up. The Windows Installer CleanUp utility window appears, listing software that is currently installed on your computer, as per the following screenshot:
(4) Select the software you're getting the "not a valid short file name" or "invalid character" message for in the list of programs and click Remove.
(5) Click OK in the confirmation dialog that appears.
(6) If you have multiple entries for the software you're getting the "not a valid short file name" or "invalid character" message for, repeat steps 4 and 5 for the other entries for the software.
(7) Click Exit.
(8) Restart the computer.
If you were getting the "not a valid short file name" or "invalid character" message when trying to upgrade or launch software, now try reinstalling the latest version of the software.
If you were getting the "not a valid short file name" or "invalid character" message when just trying to uninstall software, program files for the software in question will remain on the PC after it has been removed by the Windows Installer CleanUp utility. If you want to remove leftover program files, check in at the appropriate community at Apple Support Communities and ask which files can be safely removed.
Resolution 2
If you'd prefer not to use the Windows Installer CleanUp utility, you might instead try the Microsoft Fixit from the following Microsoft document to uninstall the program you were getting the "not a valid short file name" or "invalid character" message with.
Fix problems with programs that can't be installed or uninstalled
If you were getting the "not a valid short file name" or "invalid character" message when trying to upgrade or launch software, now try reinstalling the latest version of the software.