avg retail installation | Steps to install if you have an AVG activation code
First of all, make sure that the system date is set correctly on your computer. If the system date is incorrect, set it correctly.
Enter the activation code into the corresponding field in the Activation window. Please note that the code only contains Latin letters and numbers.
If you cannot find the Activation window or if you closed the application after installing it, run avg Anti-Virus.
And click the link Enter activation code in the lower-right part of the main window.
Click Activate.
Finally, In the window with the Activation completed successfully notification, click Finish.
Save your activation code in a secure place.
avg retail installation | How to install and activate AVG on multiple computers
Each copy of a multiple-device license for avg Anti-Virus (for example, a 3 PCs license) is installed and activated in the same way on all computers you want to protect.

In conclusion to activate avg Internet Security on all computers, use one and the same activation code you purchased.