Thanks for your reply.
When I originally tried to reinstall I was directed to a trial CS5 site, which asked for my serial number. When I entered the number I received a pop up which stated my number was vaild, but that it did not apply to any of the products listed in a box below the serial number. The products were for CS3, 4 and Adobe Design Premium and Standard. None of them were for CS5. So of course the number would not apply to any of those listed.
Looking on line for any similar problems brought me to this forum.
I had no problems loading CS5 onto my old machine, everything worked perfectly, but that was 5+ years ago.