Replying to Antivirus Installed Or Not?

ashish17's question was "if two antivirus installed on a PC", then my answer is it's quite possible. You can install 2 or more antivirus, but you can't run both of them in the same time

I still believe that running multiple antivirus programs is highly unadvised but still *possible*. In satisfying the original question, I would call it a myth that it is "unbearable" or impossible to run multiple antivirus products.

I still believe that running multiple antivirus programs is highly unadvised but still *possible*. In satisfying the original question, I would call it a myth that it is “unbearable” or impossible to run multiple antivirus products.

It's true that it can cause a lot of problems. Some AV programs block all access to a file when it detects a virus untill you decide what you want to do with this file.
However, if the 2 try the block the access to the file at the same time, things might get troublesome.

Windows Vista/7 warns you that 2 antivirus applications may conflict on your computer, which is true. It can slow down system performance due to the extra processing of files and suppose that you have files quarantined in one antivirus, another antivirus software could possibly detect the quarantined files as viruses and they would move them back and forth endlessly.

Personally I use AVG Free on Windows 7/Windows XP and Shaw Secure (F-Secure) on Windows Vista. I do not run more than 2 antivirus softwares, but I do run a few antispyware applications.

As a side note, popular commercial/free antivirus software such as McAfee, Norton, Shaw Secure(F-Secure), AVG and Trend Micro all prompt you to uninstall existing antivirus software, and some even refuse to install if it detects existing antivirus software.

I am sure it is possible but as you have heard, not practical. Antivirus software works by hooking (http://www.codeproje.../DLL/hooks.aspx) system DLLs. This is basically adding your own code between existing Windows code. In the case of an antivirus solution, the antivirus may hook one of the system32 DLLs that make a directory listing or access a file. When these particular function calls are made, Windows sees the hook and then executes the antivirus code that is attached. This way any file that is accessed is scanned before it is used by the system. As a side note, hooking has a bad name because it is also used by malicious programs such as root kits to make removing them next to impossible. Without hooking malware would have a much harder time but antivirus would also not work as effectively.

As far as I know there is no limit to the number of hooks that can be placed on a Windows system DLL. There is however an execution order to the hooks, i.e. the first program to place a hook will be executed first, the second program to place a hook will be executed second, and so on. This means that you can have an unlimited number of programs hooking a DLL which translates to an unlimited number of antivirus programs.

More is usually not better in the case of antivirus solutions. If you look at many of the reputable antivirus solutions available, they are around 99.5% or more similar in their operation and definition files. This means that you will get less than .5% gain from adding another antivirus program but you will lose 50% hardware performance due to another program accessing the file before you use it. As you add more antivirus programs, the gain goes down even more and the performance loss goes up.

Adding another antivirus program from a trade-off advantage/disadvantage point of view just doesn’t make sense. Sure you can do it but why would you? If you stick with the well known antivirus products and use good computing habits (don’t open attachments, scrutinize the downloads you open, be careful with P2P, warez, and p0rn) you should have plenty of protection. I personally use Avast! (http://www.avast.com/) and I am happy with it. It has been well over 10 years since I had a virus (mostly due to good computing habits) so a single good antivirus and not acting stupid on the Internet should be all you need.

first of all, simply try installing two of the most famous anti-virus, you will see that the first one will install normally, and the second one will tell you there is a problem and will stop, or will ask you to remove the other one.
Personally, I use McAfee on some systems and Norton on the other ones.
And I have ClamWinPortable on my USB stick. So, in case of doubt, I start ClamWin from my USB stick. So, technically, I don't have two anti-virus simultaneously installed because ClamWin is not installed (the main interest of the portable version). However, I am able to cross-check a folder with two different anti-viruses.

Is this fact that if two antivirus installed on a PC, then it is unbearable for a PC to handle it or it is just a rumor..

thanks in advance for reply..