problem

I have a problem. Whenever I dial-up to the internet a dialog pops up with the title "Messenger Service" offering me lots of porn (huh, I can find my own porn thanks very much). As the rest of my family uses this computer too, it''s only a matter of time before my mother sees it when she''s doing some banking...... Anyone got any ideas how to fix this? I''ve run Ad-Aware to no avail....