How to get rid of houseflies?

I am having a serious problem that I have not seen addressed in your column. Every year for the past 8-9 years, we have had a terrible problem with houseflies.
We get hundreds of them around this time every year. We called a company to see what it would take to get rid of them but the cost to dig up our yard and around the house was much more than we could afford so we did not go through with it. Plus, I really did not want my yard to be all dug up.
I cannot find any commercial sprays on the market. When the kitchen windows are covered with the flies I spray Windex on them as that seems to kill them off but when I kill 50 of them 60 more show up for the funeral. I see spots of fly crap on the windows and lampshades and curtains and I just can’t go on with this any longer. The washing and doing windows everyday is ridiculous. Help Help Help! What is causing them? Thanks.