Help me pls mates

I'm using Lan Network ( at College ), and there is one motherfucker keeps watching what I'm doing with my computer, 'cause each time I saw him, I could understand that thing because of the way he talked to me and sometimes when I was chatting using Yahoo Messenger, I was signed out with reason " you have been disconnected 'cause you signed in with different computer or something like that " this shit makes me sick all the time and I don't know how to handle this shit, I got zone alarm fire wall, and AVG antil virus, and I scan my computer everyday, but I found nothing, I need your help plsssss, thanks very much mate

why dont you just change your password?

Or kill the dude… and everyone else that looks at you funny.