Anti-Virus Software
 

Yes you should have only one Anti-Virus program installed.

But if you want Anti-Spyware, you Should have 2 to 3 installed.
The reason for this is Anti-Spyware works from a database of known malicious programs. Subsequently the Anti-Spyware programs are only as good as there database. The last stats I saw, the best detection for any single Anti-Spyware program was only about 40%. Those number may have changed some, but it's still a good general practice to run more than one program.

why is it advised to have only 1 anti-virus software installed?
i have AVG and mcafee total protection installed.

Ok its simply, if you want more protection get more programs that dont conflict with each other. Just dont get two different ones that do exact same thing. I've been using Avast for the longest time and I'm happy with it. Just know the more anti-(anything) software you load on your pc you will take a performance hit. Slower start up times, slower programs. anti-software eats a lot of resources.

Anti-Virus programs currently need to hook deep into the system to monitor every activity. More than one installed will almost always cause problems with only minimal gain in protection.

Anti-Virus ans Anti-Spyware programs should only be considered a second line of defense. The first is behavior. Don't open E-mail attachments, don't download or install software that you are not sure about the author, turn on automatic updates and don't visit potentialy malicious websites.

The safest way to use the internet on a Windows PC is in a virtual machine. What happens in the virtual machine environment can not cross over and affect the system running the virtual machine software.
To learn more:
Virtual machine - Wikipedia, the free encyclopedia

Virtual Machine programs:
Microsoft Virtual PC 2007
VMware: Virtualization, Virtual Machine & Virtual Server Consolidation - VMware
Virtual pc, virtual machine and multiple operating system solutions by Parallels, Inc.