Sounds like the OS isn't correctly telling the installer what you have installed. I've seen it in the past with some of the other runtimes, but not that one. Typically what users need to do is remove it from Add/Edit programs then restart the computer and run the install again. Alternatively you could try to run command promt as admin and run the command "sfc /scannow" (without quotes) and let that run and have Windows try to repair itself. If it finds anything I'd restart your computer before running the install again.