Ladies, listen to me.
If you are not expecting someone and you don't recognize the person on your doorstep, let them knock, let them ring the bell a hundred times...hell, let them stand out there and freeze. If they say they are offering phone/gas/electric/cable services, call the company to verify before you open the door. Likewise, if they are dressed as a law enforcement officer, call the police station to be sure. Keep a stout, locked door between you and your unexpected visitor. Now, I know your first instinct is to be as helpful as possible and that you don't want to seem rude. And trust me when I say that I know how tempting an offer of snow removal can be when you are knee deep in the white stuff - but if you are home alone, do not open your door to strangers. Ever.
But if there comes a day where you absolutely positively must open that door, have your gun in your hand. Discreetly, of course - because heaven forbid you become known as the quirky, asocial, gun toting, cat lady in the neighborhood.