For the longest time I had to tell my mother that those rectangles that blink a lot were not really Microsoft telling her about a virus. Or that she'd won anything...
She also didn't know what to do with the mouse when she reached the end of the mousepad.
I shit you not, my mother now uses a trackball with a mac because I got sick of it all.
I did some volunteer work in high school where I was assigned an elderly person and I had to explain the Internet to that person. I got an old lady who repeatedly tried to touch the screen of the computer to click on the icons even after I repeatedly explained how the mouse worked and demonstrated the proper use of the mouse.