I think I have a solution. Try uninstalling the program.
Next: Start>Search all files and folders> Type yahoo in the search bar.
Next: Delete eveything related to yahoo (use common sense).
Next: If Yahoo still has programs running (in the backround) use this program http://www.neuber.com/taskmanager/
Also try Cleaning the registry using :http://www.easydesksoftware.com/down.htm#Perfect
WARNING: MAKE SURE YOU BACK UP YOUR REGISTRY BEFORE YOU CLEAN IT!
Next: Download all yahoo installers (messenger ect.)
Next: Reboot
Next: Install all of your Yahoo stuff
-Note: If windows installer does not work after cleaning the registry just restore the registry.
I hope this helps!