You have to uninstall the old cs3 and checkmark "remove serial number" to erase the old serial from your system.
If you have, revisit Activation or connection error when starting CS3 or Acrobat 8 and try your old serial number again.
At the bottom of this page is "Change Region". You can then set it to United States and try again.
Best practice is to copy and paste the new serial number into the new installer. It reduces the chances of the numbers transposing.
Thanks, Gene, for your prompt answer but I'm afraid it didn't work. I changed region but I got exactly the same SN. I was very careful when writing/pasting it. No PS so far
I don't understand how the product box comes into things. There is a new installer and new serial. The product box should stay inside a cupboard, untouched and unseen...