"Hi there, my names Bob"(or whatever, then give them a card. place it in their hand and step back)
"Would you be interested in a quote for having your windows/gutters professionally cleaned?"
Then wait for them to answer. Don't feel you need to fill the gap with words.
If they say "Yes" walk round the house and give them a quote.
If they say "No" Thank them for their time, and say "If you do need a quote feel free to call me"
If they say "We have a window cleaner" Say "No problem, maybe you could give my card to one of your friends or family who need a window cleaner"
If they say "We have one but he's rubbish" Offer to quote but make it a high quote so that you have no danger of under cutting. If they are that bad they will pay the difference.
When you give them a price, dont drop your price if they try to barter. If you have to, offer them an additional service like inside cleans for 1.5x the outside and tell them it's usually 2x so they feel they are getting a discount for additional services.
Hope this helps,
Stuart