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what is the reason for this problem?

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What two antiviruses? I've done that before and had no problems. – Sasha Chedygov Apr 30 '10 at 20:45
up vote 6 down vote accepted

There are several issues. First, most antivirus programs store a signature - blocks of code from viruses - that they are looking for. It is possible for antivirus A to detect the signature file from antivirus B and consider it a virus and kill it. This was a little more prevalent awhile ago, not so much these days.

More importantly, current antivirus programs link into the operating system at very deep levels and monitor all disk I/O and network I/O and scan the streams of data for signatures and suspect activities (such as calling a format command.) Having one of these usually degrades your system from 10% to as much as 30% or more. Having two of them inspect all traffic and I/O - one after the other - would be very degrading.

The other thing is that once upon a time, the antivirus programs would scan their links into the operating system to make sure nothing got in under them. If antivirus A detected something got ahead of them (like antivirus B) they wouldn't like it and would try to establish a closer connection under B. B then would try the same thing and you'd have this subtle war in your OS. Not sure if this is still the case as the manufacturers have been getting better about this.

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"degrades your system from 10% to as much as 30% or more" Citation please.... – MrStatic May 1 '10 at 2:14 Compares start times, file opens, Windows apps times w/various antivirus programs. There are others, but still searching for available stats. – Blackbeagle May 1 '10 at 2:40
I should qualify the 10-30% - this is BOOT time, program start time, file opening... If you start say a word processor, it will be slower to start. Once loaded and sitting on a blank page, there shouldn't be much problems while typing. When you save, again you'll take a speed hit. I don't mean to say the computer will be degraded at all moments, only during I/O or during scans. – Blackbeagle May 1 '10 at 3:44

Having two anti-virus running at the same time is simply the best thing you can do to have problem and overall dramaticaly slow down you computer...

You better have one good anti-virus (I'm talking about Kaspersky, Nod32) instead of 2 poors anti-virus together..

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I myself use Nod32 and of course it's faster than kaspersky!!but i want to get information about this issue! – Nobody Apr 30 '10 at 20:54
getting information about what? My advice is not to use Nod32. it deletes an infected executable, but others (Kaspersky, Avast, ...) cold successfully clean an infected file. – sorush-r May 25 '10 at 19:11
@Sorush: an executable can't be cleaned (99.999% of the time) without severely breaking something, and it's better just to delete it anyways (as no virus protection solution is perfect). – squircle May 25 '10 at 19:18
@ thepurplepixel: of course, but there is degrees of success! – sorush-r May 26 '10 at 15:17

Avira and Avast seem to get along fine. You can read about my experience using both.

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You could avoid some of the potential problems of multiple AVs by using GData, which combines the AV engines of 2 other commercial AVs and has a very high detection rate. It's very large and not free though.

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Antiviruses are generally virus-like programs: Because their behavior (not their goal!) is very similar to viruses. they control almost all of user and system activities on your computer, so they should have a deep access to your operation system's resources (threads, processes or even registry data). viruses do like that: they should have a certain access to OS's threads for implementing a planned attach on resources. (this is a very general scenario)

Nowadays, anti-virus softwares take an artificial intelligence approach for control and detect harmful or potentially dangerous processes. (instead of using a virus repository or database as main protection technique). This fact makes them more dynamic in virus detection: not all known viruses but most of new and unknown viruses could be detected by analyzing their activity. of course it could hurt targets of some trusted processes too.

In this situations two protection program could not work together.
as an example in a specific scenario suppose that two anti-viruses, (A) and (B) are installed on the same OS. (A) will detect (B) as an 'unexpected' or 'unwanted' activity (because it behave like a virus, as explained above). And will try to access/monitor/control/kill processes of the (B). also (B) detects (A) as a process that tries to stop (B) so (B) thinks that (A) should be a virus (anti-viruses defend themselves!), and vice versa.

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