Freezing Desktop

I’ve volunteered to help a number of Senior Citizens use computers. Anyone who has had to deal with someone who knows nothing about computers knows how frustrating this can be. My main problem is not with the personalities that are hard to teach, but with what they can do with the desktop arrangement. I had one 85-year-old guy overlay three toolbars that covered the whole screen. At first I had not idea this could even be done.
How can I make the WXP desktop fixed? Of course I can give them limited access but how do I configure the desktop to only open or close the selected program and not let them move, delete, or change any file or icon in their profile?