Q. Is it rude for a guest to ask to use a host's computer to check e-mail or surf or play games?

A. Why would a guest ask to do this? Their desire to disassociate from the social interchange is not a good sign of their mental wellness and, of course, it is downright bad manners. Would you let a guest try on your clothes? Asking to use someone's 'personal' computer is like asking for a piece of paper from someone's diary. Of course, he or she who wants to use your computer is clueless and will probably need a major explanation of why you are saying no. This may also include professionals of great self importance like doctors and lawyers who think their emergencies are your emergencies. They aren't.
Why don't you say something along the lines of:
'Let you use our computer and deprive us of the pleasure of your company for even a minute? Heaven's no.'

Questions for William Norwich