Instant Messenger – Annoying

For a variety of reasons, I am NOT an IM user. Recently, I mistakenly clicked the IM icon on my taskbar (placed there by my employer’s computer setter-uppers – its a work machine – XP Pro). I closed the resultant window, but now, whenever I boot IE or FireFox, up comes an IM loggon window!!! How do I get rid of this annoyance?