I'm sorry for coming in late to the party, but could you be a little more specific, C-Los?
Are you saying your SBC/Yahoo service showed six new Email accounts? If so, this could be done from anywhere in the world with just your username & password, however they could have been obtained.
If you are saying that your computer shows six new Windows user accounts, THAT's another story and could be a sign of someone nosing around inside your local network, possibly through your wireless connection.
My own yahoo.com web-site account once had a unknown username & Email address show up in it which alarmed me, but I deleted it, it never came back and I stopped worrying about it.