What is the harm of having two anti virus scanners running at the same time?

[Zack]

I don't understand what the issue is, I would never do it because...the obvious notice that it gives you "It would not function well" < or something of that sort.

But what is the real details? I installed Avast! Home Edition (For the 5th computer...I can't thank you people enough) but this time it was on my father's computer after we got a virus from MSN (that was NOT detected by Symantic AntiVirus)

Now My father does not want to disable symantic, and wants both of them to run at the same time, it does say the service is running, but I am wondering what is the harm? is it just false reports? will it not work at all?

PS: It is not a regular Symantic , it seems to be one of the old classic kinds..(free ? maybe?) our local university has them free of charge for those who want them, with no activation what-so-ever so I assume it is freeware.

[onlysomeone]

I'm no specialist, but i would say if you run two antivirus-programs they every times scan each other...
That means:
you run a exe file -> avast scans it -> an other antivirus detects avast scanning the exe and scans the process -> avast detects that the other antivirus scans the process and scans it -> again the other antivirus detects the scan of this and so on...

so they scan each other and conflict and can cause a hanging system

also they can block each other or can block the internet connection because both try to scan the traffic...

yours
onlysomeone

[DavidR]

Over and above what has been said.

Resident AVs load low lever drivers that hook files so they can be scanned before they are run, these can clash and cause a conflict, which could lock up your system. If this happens during boot then you could possibly be locked out of your system.

You may well be able to get in through safe mode but then you have to start resolving the problem with will probably need at least one AV to be uninstalled, you may even need to uninstall both and run other tools to ensure all remnants are gone, before reinstalling one AV.

There are a lot of could, may, etc. in the above but I for one even with a good back-up and recovery strategy will only have one resident AV on my system.

Disabling the other AV simply isn't enough and Symantec/Norton is probably high up on the list for possible conflict with other security applications (even when uninstalled needing other tools to completely remove), you only have to browse the forums to see the detritus left behind after a NAV uninstall.