I have to write to a widowed woman in Great Britain whom I don't know and she doesn't know me. I don't want to be inpolite and are wondering what the correct personal address is. Can anyone please help me?
First, you must find out what family name she uses, her former husband's name (Windsor) or her maiden name (Stewart).

Then address her as Mrs.Windsor or Ms.Stuart.

Also, I don't think you'd be wrong with Ms.Windsor either.

This is what I would do, but I'm not British. So you may want to wait for a more culturally sensitive opinion.