Serial Product Number won't work

[Anon89]

earlier on today i was trying to download somthing which needed my serial product number so i went on google to find out where it was (can you tell am not very computer smart??) finding out it was at the bottom of my laptop. i carefully wrote down my PSN and typed it into the box double checked it was right then clicked ok, to my horror it said incorrect so i rechecked everything to make sure i was right, typed it in again and as i thought it was wrong again. could anyone shred some light on the matter ?? thanks x