I always heard on these forums “Never use two antiviruses on the same pc, they will crash the pc.” Well, today, my buddy asked me “Why does that happens?” and I don’t know how to answer it.
Just for the sake of curiosity, can somebody explain me how does two antiviruses crash the PC?
Polonus has a good analogy, which is that two AV’s on a computer at the same time are like two dogs fighting over a bone. (The bone is the file which the computer is reading, writing or transferring at any one time, and which both AV’s attempt to ‘grab’ at the same time in order to inspect.)
It’s usually conflict of the drivers.
Imagine, for example, than an application is trying to read a file. Antivirus A intercepts this file access and starts to scan the file. To do that, it has to read it first… and this file access is intercepted by Antivirus B - which starts to scan the file itself. Now, this file access is intercepted by Antivirus A again… and so on, in an infinite loop. The result - system freeze or crash.
The conflict may be more subtle, appearing only in specific situation (e.g. when a virus is really detected). There are various scenarios (Vlk came up with a nice example once - where two antiviruses installed on the same computer resulted in a virus, normally detected by each of them, actually getting undetected at all).
Hmmm… So one antivirus starts scanning the other’s proccess… Yeah, I guess the RAM gets eaten up after some time 
I wouldn’t say only the RAM but the CPU always calculates the same on and on and that forces errors…