Just a note of caution here - Two anti-virus programmes running in the same computer can be worse than one, and can leave you partly unprotected. If they both fight for control of the interrupt handler for example, you could end up with the interrupts being sent to the wrong handler routines.
What on in a case a system has a default antivirus and you install a new antivirus because you are jot satisfied with the service you get from the former would it also affect the system?