Hello, Kelly.
I regret for any inconvenience caused. Please try to use a different web browser and check if you can access "Check the box". This issue does not impact the functionality of your previously installed products, nor does it affect the installation or activation processes of newly purchased AVG products.
Click on this link http://support.avg.com/SupportArticleView?urlname=Activate-AVG-Internet-Security on how to activate AVG Internet Security - Unlimited (1 year subscription) on your PC.