Are you with the client as they are viewing your slideshow? In my experience, there is a good possibility that a non-computer savvy person will not be describing the problem correctly. It is best to see what they are seeing, without their interpretation.
As long as the web browser has Flash installed (not all of them do by default), the show should be viewable. Of course, other system settings may affect the enjoyment of the show (such as the global volume level, which is not something the Flash show can control), but that's for the individual to determine what about their system is different from the expected norm.