I use yahoo email and the notifier seems random on my computer - sometimes it's there, sometimes it's not - have really tried to find out but can't seem to crack it. I also get the two note tone when something other than an incoming email is going on. Oh, have just thought maybe that is when I get spam - will certainly try and check that out myself. Sorry to be a nuisance.
carmalee - you need to sign in to yahoo messenger to get the alerts. Sometimes it kicks you out if you sign in on other messenger systems afterwards, just log in again to yahoo messenger - works for me