An anti-virus program is a very advanced piece of software specifically crafted to detect advanced, sneaky, irritating viruses which are designed to hide deep inside the operating system. Anti-virus tools themselves are sometimes designed to be sneaky to help find these viruses. Due to this fact, the presence of two anti-virus tools, could lead to confusion, false detections and chew up significantly more CPU resources and therefore may slow down system performance, than if it were running by itself. Most anti-virus tools also have a database with virus "signatures" in it. Some anti-virus tools may think that this is actually an infected file and may force the user to do things which could later destabilize the system. Thus its advisable to stick to just one anti-virus program for your PC to respond to it better.