Help! My boyfriend is able to see every move I make online. How can I tell and/or stop it?

He is able to see my e-mails, chats, and internet history even though I have it set so that it doesn't save the history. He does this every time I go out of town or he has a long period of time without me home. I even have a password set to log in to my computer. He's quite tech savvy but I'm rather baffled, please help!