So I have always gotten jokes. A friend told me his favorite and i just don't understand it. I did a web search and found variants but no explanation why it's funny or what it means. Can you help?

Cars have windows. Houses have windows. So it can't be windows that makes the car go.

I swear I don't understand, and he tried to explain it to me. He said it's a double meaning with Windows the operating system but I just don't don't don't get it.

Can anyone make this understandable to me? I may have screwed up the retelling, because honestly I have no idea what the hell's going on with this joke.