Okay, let's see if I can get the ball rolling here...
First, you Do realize this is a user-to-user forum I hope. So, with that caveat;
It could be that, like me, nobody knows what you mean by an invalid "Cupon Code" (do you mean Serial Number?).
Where in the installation does it ask for it?
What, exactly, does the error message say?
Beyond that...
Could you tell us what OS and version you ara running, and maybe something about the machine (memory, etc.)?
Where did you get the CS5 you are trying to load?
--OB