http://www.techsupportforum.com/f174/is-your-pc-running-slow-247566.html :Now, one piece of advice that applies equally to AVs and firewalls. You only need one of each! Having more than one firewall and AV can cause possible conflicts and system instability. Although you may think you have more protection by having more than one of each, it could cause you problems you were not expecting.
Never use more than one Anti Virus. Although it might sound like a great idea to run two or more, in reality it’s not. AVs usually have a ‘real time’ monitor that helps protect your system. This monitor will want to have a look at any file that changes or has been added to the system. If you have 2 AVs then every time one looks at a file, the other AV will think that file has changed so it will want to take a look as well. Now the second AV thinks that file has changed so it wants another look. So, of course, the first AV thinks that file has changed….you get the idea. You could end up with an unstable system, a really slow system or unexplained crashes.