how to configure viewing enviroment in adobe

[#1]

dear when i open pdf file so i get the given message 

The document you are trying to load requires Adobe Reader 8 or higher. You may not have the Adobe Reader installed or your viewing environment may not be properly configured to use Adobe Reader. For information on how to install Adobe Reader and configure your viewing environment please see http://www.adobe.com/go/pdf_forms_configure. This is the message I get when trying to open a simple document. What can I do?

[#2]

dear when i open pdf file so i get the given message 

The document you are trying to load requires Adobe Reader 8 or higher. You may not have the Adobe Reader installed or your viewing environment may not be properly configured to use Adobe Reader. For information on how to install Adobe Reader and configure your viewing environment please see http://www.adobe.com/go/pdf_forms_configure. This is the message I get when trying to open a simple document. What can I do?