I downloaded comodo onto my windows7 OS, and then attempted to download the free comodo anti-virus, and it wont let me, and gives me a message about incompatibility.
If you already have Comodo Firewall installed the correct way to install Comodo Antivirus is to go to the start menu and select add or remove components. You can then install the antivirus along with the firewall.
Let me know if you have any questions.