25F and Stickyb Thanks for the advice.
But what is causing the crashing, particularly when my son plays his memory-hungry games? Is it the lack of User memory? If so, how do I increase it? I have tried putting an extra stick of 512MB alongside a stick of 128MB but this caused more problems than it was worth. Perhaps the new stick was faulty, perhaps it was out of balance with the smaller stick?
Perhaps a new computer is the answer. I have spent five years fiddling around and adding bits to my present machine. Perhaps I should start afresh.