First, Ex_Brit is much more knowledgeable than I but I will pass on a Windows tool that could help you.
If you go to http://support.microsoft.com/kb/290301
and download the Windows Installer Clean Up Utility that might clear the way up for you to reinstall the program in question. __Read the article closely and _only_ click on the specific program you wish to remove._
I used this Windows utility as it was the only way to remove an older version of a Dell computer utility (Dell Support Center) and install the newer version. That Windows utility worked for me in that one instance.
From that MSKB article ........When you are working on your computer and installing a new program, the installation suddenly fails. Now you are left with a partly installed program. You try to install the program again, but you are unsuccessful. Or, maybe you have problems trying to remove an old program because the installation files are corrupted.
Do not worry. Windows Installer CleanUp Utility might be able to help. You can use the utility to remove installation information for programs that were installed by using Windows Installer. Be aware that Windows Installer CleanUp Utility will not remove the actual program from your computer. However, it will remove the installation files so that you can start the installation, upgrade, or uninstall over.