Why does my screen suddenly change when I move the the cursor with my touch pad, as though I have clicked it?

You have a laptop? I had that problem with mine. When I called the tech support people they found out that my mouse had been installed as an external mouse instead of the touchpad. Once they fixed that, it was fine. You could probably do that yourself by going into the mouse part of your control panel and making sure it's not programmed to be the external mouse.

I have lop top and I have same problem ,because you click on the mouse pad with your finger is very sensitive,if you like works only by click you have to go to mouse update,and fix from there.

When you tap (or put accidental pressure) on the mouse pad of your laptop it responds as a mouse-click. This is a design feature. I'm sure you can disable it, refer to your manual...

Time to buy a new mouse or clean your old one!