Sounds like you are using Yahoo mail which has a built-in messenger and have the Yahoo messenger installed additionally. When Yahoo detects that youรขโฌโขve already signed into Yahoo Mail, it will disconnect you when you try to sign into messenger or vice versa - hence the message that you are signed in to another computer. To get rid of your problem simply disable chat in your mail (!) account by clicking the smiley and following the prompts. Yahoo remembers your settings until you change them.
http://help.yahoo.com/l/us/yahoo/mail/yahoomail/chat/chat-05.html