Other people reading your IMs? help!?

ok so I think that someone else has been accessing my AIM messages on my computer but I am not 100% sure. So I have a few questions. Is there any way possible that someone could be doing this (especially if they know my pword)? Is there anyway to find out for sure if they have been doing this? And what can I do to stop them (or at least delete ones I dont want them to see)?? Thanks a bunch!