It is not allowed to install several anti-virus programs on one PC at a time — they will conflict with each other that may considerably slow down you PC or make it completely non-operational. If you have an anti-virus software already installed on your PC, but intend to install a new product from another vendor, then you need to remove the anti-virus program you have, prior to the installation.
If you wish to install another anti-virus program, you do it at your own risk, and become responsible for all the consequences because the consequences of such installation might be unpredictable including inability of your OS to boot as a result of two or more anti-viruses installed on your PC.