Trying to activate my renewal of AVG PC Tuneup (12 months - 1 computer)

While trying to follow your instructions for activation of my product, I get a message saying that I have another AVG product installed on my computer and it is incompatible with the one I want to install.  How does that happen?   What can I do about it?  It's VERY frustrating.  Your instructions don't fit either because when I click on AVG help there is no "activate product" showing in the drop down menu.