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Why is it bad to have two anti-virus systems? It could be useful to get better detection rates. In the second post from the top of the page Stranger claims it's not bad to use two antiviruses as long as they don't conflict. How do you know if a-v's don't conflict?

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So you have found one place that carefully says that having 2 AV's might not be bad. But you disregard all the other sources that claims it is bound to cause problems? –  Nifle Jun 18 '12 at 7:54
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No one's ever given me reasons to believe 2 AV's are bad. –  Celeritas Jun 18 '12 at 18:34
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Antivirus buries itself so deeply in the OS anymore that eventually the two packages will get in a turf war, have a rather intense knife fight and your computer as the innocent bystander will end up in the computer Hospital ER bleeding its bits out. The Cold War comes back with full nukes if you ever get McAfee and Kaspersky to install in tandem... That's why they always check on install for incompatible security software anymore and prompt you to remove one. –  Fiasco Labs Aug 27 at 3:58

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Two active scanning anti-virus programs will conflict. Passive scanners won't, and you can have as many of them as you want.

The issue is the way active scanners work. The behavior they're looking for is programs that attempt to manage the entire system and intercept and mangle the data going to other programs. However, they prevent this behavior by engaging in precisely this behavior. To an anti-virus program, another anti-virus program behaves precisely as a virus does -- monitoring other programs, intercepting and filtering their data, and so on.

If either anti-virus program allowed information to get to a process without scanning it, it wouldn't be doing its job. But they can't both scan the data (since whichever wasn't last would have scanned the data that went to the other anti-virus program, not the data that went to the target process). So there is no sane resolution.

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And the end result of this is that everything slows to a standstill as a fileopen activity in one triggers a fileopen for scanning in the other and so on. –  Rory Alsop Jun 18 '12 at 8:09
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It can also just deadlock. A program opens a file. Antivirus one stalls the open until it can scan the file. Antivirus one then opens the file to scan it. Antivirus two stalls antivirus one's open until it can scan it. Then antivirus two goes to open the file to scan it. Antivirus one stalls the open until its scan can complete. But it can't make progress because antivirus two stalled its open. Now each antivirus program is waiting for the other to complete its scan so that it can begin its scan. Oops. –  David Schwartz Jun 18 '12 at 8:23
    
Does that mean it's bad to have an anti-virus and an anti-malware? For example Norton AV and Malwarebytes? –  Celeritas Jun 18 '12 at 18:37
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@Celeritas: If they're both active scanners that run all the time and monitor network or disk access, then yes. (This is why I only recommend the free version of Malwarebytes, which is a passive scanner. It's not good enough to be your active scanner, and you can only run one active scanner. But it's a great passive scanner.) –  David Schwartz Jun 18 '12 at 19:03

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