Antivirus programs have had, over time, like the threats they attempt to counter to become more and more invasive into the operating system. That means essentially that parts of them need to be loaded along with the operating system. At this stage they are not designed to coexist - the other co-existing entity could be a threat. The antivirus designers do not get together in some back room and cozy up to each other and find ways to coexist, why should they? They are, after all, business rivals seeking to be our chosen product.
That means you, me and all the others users pretty much get to make the choice of which single antivirus program do I want embedded in my system now. We do not get the choice of which pieces of various antivirus programs we want, any more than we get to choose which pieces of which operating systems we want running together in our system.
Look at it this way, you cannot test drive two cars at the same instant. You get into one, test it and then get out, you then get into the other and test it. You then decide which one you want to drive everyday.
