Re: Can't install adobe Reader - Error message 'Key not valid for use in specific state"

Assuming that you are in Win (which you haven't said), you can try using this tool to remove all traces of previous versions of Reader:

http://labs.adobe.com/downloads/acrobatcleaner.html

Then, download the full offline Reader installer from 

http://get.adobe.com/reader/enterprise/

and run it immediately after restarting your computer.