You should be able to just install/upgrade right over it.
If you haven't fixed this yet, Microsoft has a Fix it for inability to uninstall programs. You can search and find it independently, or you can download the Fix It Portable from - please double-check and take due care and don't take my word for it - this location at Microsoft.com http://go.microsoft.com/?linkid=9775982
That's the entire thing but it's just a stub that runs an installer that downloads the rest. I'd recommend using that to install Fix It Portable to a USB drive (or similar) so you can use it wherever you need it. You only need to click through the fairly slow "I can't uninstall an application" and let it find them and then click to remove them. I can't give you perfect menu options because MS changes them occasionally.
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