My computer has been infiltrated via my network!!!!!?

I rent a room from an old friend. The modem for the house computer runs through my room, so he said it's no big deal that I use his internet access. But now I have been using it for some time now and it seems like he is accessing my computer from his computer. Emails and Facebook notifications are read before I even open up my accounts and he seems to know stuff he shouldn't, almost as if he was reading from my mail and invading my privacy.. I am trying to figure out how I can keep his nosy *** out of my business. I have changed all my passwords, but that doesn't seem to help. Yahoo told me already that my email was compromised and I don't think it stops there. Can someone help me with some useful tips and advice?