Many of the better AV pgms. provide protection in many aspects of PC operations. If they're doing their job it can be intensive "watchdog" of all your PC going on. In this aspect dual install of AV simply step on each other to provide the same services. In fact some AV pgms. upon finding another AV, may "disable" or reduce their effective operation regardless, so it has the sole task of providing protection. Even when AV pgms. are working for the time being it can easily fall in dispute and cause issues of their own nature. Further, the system resources can be hogged and thus reduced for PC operations when needed. It has always been good advice not to install similar AV protection. However, there are times when one area of protection can be allowed to work in lieu of the what maybe provided only because it is optional and I find "firewalls" are a good example of this.
To better resolve the issue just visit the Av support website and they in turn usually suggest the same advise. Further because provided AV is free for 36mos. no need to defeat it;s use.
I recently purchased a Dell T1600 64-bit computer running Windows 7 Pro, which came with a Trend-Micro Worry-Free Business Security for 36 months. This is the 10th computer that I've ordered for my organization like this and have had no problems. But recently a new tech set this one particular unit up and installed AVG Free security software on this unit,(Why? I don't know!) then brings the unit back to me saying that it is not working properly and should be returned. After informing him that you cannot have 2 AV programs running in the background I am asked to provide proof. So I ask you my fellow Cnet people. Help by giving your comments so that I don't have to go digging thru all my junk at home to find that first Basic Computer Book from my 1st yr. of schooling. Thanks.