Because I don't like to ask questions and not acknowledge the replies, I take this opportunity to thank all those with sensible replies. As it turned out, B2's knowledge of computers is so limited that she didn't realise the "operating system reinstall disk" was what I was after. I asked if she had a Windows XP disc and she quite honestly answered that she couldn't find one.
All systems are now go, but perhaps those who advocate changing operating systems should stop and remember that there are still those to whom a computer is a box which works when they follow steps 1, 2 and 3. If it doesn't work they take it to an overpriced technician to fix. That's life. Linux won't change anything for them.