Yes, that did help! Thank you. I changed the theme from the default Brook to Snowman, then PC crashed, as usual, when adding third menu. Then I changed theme to Wedding, and success - I managed to add 3rd & 4th menu.
I have one more menu to add (they are all very small), and I was checking that all the correct photos were in the file that I wanted to choose, There are a few photos plus approx 5 short videos that were taken with a normal digital camera, and that I have previously watched via WMP. However, I have just tried to open them in WMP and they begin to play and then WMP immediately shuts down! So I seem to have a new problem.
Why would WMP quickly close down when viewing a short video?