Somewhere some software install must have died. Now when I try to install software, I get popup message from the Windows Installer saying "Another program is being installed. Please wait until that installation is complete, and then try installing this software again.".
A search on serverfault brings up "Windows Installer Cleanup Tool" which Microsoft has stopped supporting. Another search lead me to WiseFixer which wants $50 to fix my computer.
I don't mind paying some money to fix my computer but I no longer trust things I find via most product review sites or web searches.