Hi DrGaryG
Open Regedit, navigate to:
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall]
and click on the + sign to open the key.
You'll need to go through the list to find the offending entry for Yahoo
Mail. Some of the entries will be cryptic sets of numbers and others
will be plain text entries under the name of the program. Check the
"plain" entries first, and if you don't find it there, go back and click
on each of the cryptic entries in turn. You can see which program they
represent by looking at the "Display Name" value in the right-hand pane.
When you find Yahoo Mail, delete its key from the left pane and you
should be all set