Internet Pirates

My dad got a call today from Cablevision (our internet provider). Somehow somebody's been tapping into our IP address and using it as their internet connection too. This apparently has been going on since November or so. I'm curious to know how they did this. My dad is of course paranoid that his information he keeps on this computer has been compromised. Needless to say, I will be backing up my files to my flash drive as will he and we will format the hard drive and start from scratch. Gee, this gives me an idea (insert light bulb). What a great topic for my next speech in English class. Don't mind me, I think out loud sometimes.