Borrowing or stealing?

I just found out that my roommate has been using my computer without asking me. Whenever I am out of the room she goes on my computer and uses my Microsoft and my printer. When I confronted her about it she told me that I was overreacting because I had let her use it before, when she was in a hurry to get to class with and needed to print something out fast. I told her that using my computer when I am in the room and have given her permission is totally different then using it when I am not in the room and not giving me a heads up before or after she uses it. There are free labs on this campus and I know she knows where they are. I do not understand why she feels the need to use mine. She has not offered to by inc or paper to refill what she has used and I am not sure that I believe that she is going to stop using it. But she doesn't know how to affectively exit out of word... It's tricky on macs, so I guess we will see how it goes.