I'd suggest downloading
RevoUninstaller. Use this to uninstall instead of going through Add/Remove programs.
Once you start RevoUninstaller, it will populate a screen with all your programs. Choose Yahoo and click uninstall. It will ask for what level you want to look for with regard to files that may be left over. Choose #4. Then go ahead with the uninstall.
Revo will first scan for all the files/folders/entries it can find for Yahoo. Then it will attempt to use the built in un-installer to remove it. If this fails it will continue and Revo looks to see if there are any remaining files, folders and registry entries on your machine. If so you will be given the lists of what remains. Choose Select All on each of the final screens and Delete for anything it finds.
This is a great program, especially for times when the built-in un-installer of a program fails to remove it. As long as you can see the program in Add/Remove programs you can use Revo to get rid of it. I've used it several times for failed un-installs, always with great results.