Depends, what instant messanging program are you using? Do you have a router/firewall? As far as I know, no - people cant see what files you have open or where you are on the internet. But if I rememeber correctly, Windows Messenger does have an option to allow other people to view whats on your monitor, but both you and the person your talking to have to press "I agree" and you can exit it anytime by pressing Esc...