Simply launch the registry editor (type regedit at Run command), and navigate to the following registry branch:
Then, on the right pane, right click on the registry editor and select “New”, then create a new DWORD registry value. Name the new registry key as Plural, and assign it the value of 1 (decimal). If some case, the value of 2 should also works. To disable or disallow multiple instance of Yahoo Messengers, simply change and set the “Plural” registry key’s value to 0, or delete the “Plural” key.