Yeah, don't apologise.
Stick on the biggest grin you can muster, and say......
1) Hello! I'm Marc, from StockClean, we have a window cleaning round in your road, would you like to be added?
2)Good morning sir/madam (don't ask how they are, many don't like that if hey don't know you) would you like a window cleaning service?
Its really that simple.
Remember when they open the door they are thinking already on how to get rid of you before you have opened your mouth, don't babble, don't ramble, get to the point. Do the call, get your message across and move on.