Even I too got the same error when I am trying to install AC 21 in my computer.
I have installed that last time but later I uninstalled because of the space in my system.
When I am trying to install it now, I am getting that error.
In morning I updated that computer after that again I got the same message.
What is the reason for that?
From morning am trying to install that its not happening
Roopa A S