If he set up the computer before he gave it to you, he probably did. I wouldnt use it at all until you reformat it. In fact, dont tell him you reformatted it, and see what happens. Or just start visiting a bunch of kink sites...
First, disontinue use of the computer. Then, approach him about it, explain why you believe he bugged your computer and ask him why. He has no right to intrude on your personal doing, even if he is your boyfriend.