Multiple Antivirus software in a single PC

There are a few number of free antivrius (AV) software like Bitdefender, AVG, avast! etc. As no single AV software can detect 100 % new viruses, some might think that having two AV software in a single PC can complement each other to offer higher protection. This is not the case. AV software packages are not only incompatible with each other but also confuse each other. If they were made staying memory resident together or performing virus scan, the PC will slow down and might even hang.