My husband and I were the victims of a home invasion about 18 months ago. After turning this wacked-out young woman away from the front door, my husband got on the phone to 911 (who were more than useless, btw).
I was (surprise!) in my sewing room along the side of the house. I saw her come walking by, not knowing what had happened at the front of the house.
To make a long story short, she pulled out a 15" knife and proceeded to slash out the window screen. I jumped up, closed the (double-paned) window, which she then broke through with the knife and climbed in.
We were never hurt, thankfully. By the time we got her out to the front yard -- she kept saying that she wanted my car -- the police showed up and took her away.
We learned several things: Yes, Martha, you CAN break a window with just a knife. Said window glass will get into EVERYTHING in the room. Alarm systems work really well when you remember that you have them (ours was only a week old and we completely forgot about it!). A can of mace, an axe handle, or even a fire extinguisher are all very good defensive tools. We keep one of each by every door. (I won't have a gun in the house, no matter what; sorry)
At some point, we would have been better served to just have thrown the phone on the floor (with the 911 operator still talking and asking stupid questions) and dealt with her on our own. We just kept backing up and trying to keep her at a distance.
The suggestion of the car alarm button is a good one, but only if your bedroom is close enough to the car.
Oh, the kicker is that she has *yet* to go to trial... :-[