[soko]

Have you looked through your running applications? There are remote desktop applications out there where a user can install a little non-virus application on your computer then is able to log in and manipulate your screen as if it was on his own desktop. They aren't intentionally malicious and most have install/uninstall applications.

It's also possible that your friend is formatting text to make it appear like it's your name then sending it along with his own message to appear like it's you, when it's really him. I can show you an example quite easily.