Today I couldn't register a new Notification Services App on my machine -- Try as I may, I got an error that looked like this:

Microsoft.SqlServer.NotificationServices "Required information could not be retrieved from the registry"

I went ahead and re-ran SQLRun_Ns.MSI in an attempt to "fix" my installation, and that just changed the error:

The Notification Services registry tree has not been created. Reinstall Notification Services or repair the installation.

Finally went into control panel, Chose my SQL install and then Notification Services, and found that neither the Client or Server components were installed (even though I see some of the files on my machine)....chose them (again), completed the install, and all is well.