Help please - Registration product code

First, I am sorry for my poor English skills.

About four years ago, I bought Acrobat Reader 9.

Recently, I have replaced my laptop, therefore I tried to register - using the code I had.

But I couldn't register the code because already registered.

'The code you entered is invalid. Please try again.'.......

I want to I know how register the code.....

Respond to the request, please reply & E-mail.