It is well known, that you cannot have two antivirus tools on guard, because of highly undesirable interactions.

But there is a view that you can have two of them, provided that:
only one is on guard, and
you only use the other one for scanning on demand, perhaps after first disabling the one on guard.

From what I found, AV developers just discourage the use of their product with another one, without specifying other factors or conditions.

But what is the view of the board’s experts?

No .

I second that! Even if online active scanning is turned off or it makes you think so. The modules that load into memory use ram and CPU cycles.

For extra coverage use Virus cleaners that run and exit, and that are redownloaded each time needed.

Use them after a Virus incident from your active online Virus scanner to check with different and sometimes bettter eyes!
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Some powerful Virus cleaners

Nod32

Download http://finalbuilds.edskes.net/nod32.htm
If the above link if it fails go to http://home.hccnet.nl/h.edskes/mirror.htm

Slide down near bottom of page find nod32, to the right will be 3 Mirrors marked Online try each one of them will work.
Boot to,Safe mode only to run.

Before Scanning click Setup and click all boxes under Scan typically only System memory is not checked. So check it. Then click logging, Then Scan and clean.

It is very thorough and may detect some other malware cleaners as a threat so if it seems to point say SpyBot then click Leave.
If you have doubt about and issue then Quarintine it and it can be restored.

Depending on CPU and HD speed and the fact we are in (Safe Mode slower also) it could take a while.

Norman Malware remover http://majorgeeks.com/Norman_Malware_Cleaner__d5450.html

http://majorgeeks.com/Dr.Web_CureIT_d4783.html

http://majorgeeks.com/Kaspersky_Virus_Removal_Tool_d4515.html

http://majorgeeks.com/McAfee_AVERT_Stinger_d4063.html

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There is one Virus scanner that I like that was designed just exactly in answer to your question to run with any Virus scanner.

ThreatFire it has been getting better and better.

I have been using ThreatFire for more than a year, it just went from ver 3 to ver 4.

It was designed to be used with and to co-exist with other Virus scanners.

Additionally it uses a totally different process to protect. While conventional Virus scanners work from definitions ThreatFire works on recognizing Virus/Malware activity.

It's like looking at it with 2 sets of eyes and from a different angle.

It works like some Firewalls do to learn what is good/bad.

After install it will ask you about everything that could be a security issue. For example the first time you run IE or FireFox it will prompt you. You would answer to approve and remember the setting. From then on no more prompts about IE or FireFox unless the exe changes like in an update.

As it queries you about the prompt to help you determine to approve or not you can google it with one click.

http://www.threatfire.com/Download/

Mike

Thanks for the replies.

@ mflynn
A particularly comprehensive analysis, much more than I expected! I have saved all links and advice for reference and will proceed with (at least partial) implementation.

As a result of the many points raised, I now have further questions. I hope they don’t sound irrelevant, I have learned that I can prevent a lot of surprises by informing myself well before I try something intrinsically risky.

1. Are the Virus Cleaners similar to the on-line scans you run over the internet? Most of these require Active X controls and the IE, which had made me rather apprehensive. I try to avoid using the IE and Active X.
2. How is it that the above on-line scans/Virus Cleaners don’t interfere with the resident AV? Is it because they run in safe mode?
3. If it is OK to scan with a Virus Cleaner in safe mode without uninstalling the resident AV, why is it not OK to scan with a second AV in safe mode? In this mode the resident AV should be fully inactive.
4. The particular additional scanner I had in mind is the BitDefender 10 Free Edition which, being free, only scans on demand and cannot be placed on guard.
5. When you say the Virus Cleaners “run and exit” does it mean they uninstall themselves, or do you have to uninstall them yourself afterwards? If they are not uninstalled, won’t they interfere with the resident AV?
6. If you quarantine something found by a Virus Cleaner and it is then removed, how can you eventually restore the quarantined item?

in short - NO. Antivirus on demand are generally online scanners. but to have TWO local scanners, you'll end up with conflicts.

As usuall, I'm the contrarian on this subject.

As no single product is the 'silver bullet' we all would love to possess, I am of the view

you can never have too many tools.

The caveats noted above are real issues.
Just like you can only have one firewall active, you must ensure that only one A/V is
in memory at a time (ie actively 'on guard' as stated above), but that is not to difficult to manage.

You can't just blindly install with the default 'typical install' option and need to control
what options are enabled, (eg: disabling Link Scanning in AVG), but having
multiple tools is always possible.

What would you think of a carpenter with only one tool in the tool box?

1. No

2. Who said they would interfere with the installed Virus scanner, the installed virus scanner would likely interfere with them.

But these are designed to work with Online active Virus scanners. The most I have seen them do is when they find something the online Virus scanner will sometimes activate even if it had just completed a full scan and did not find this issue.

3. Because the cleaner is not installed it runs from a stand alone exe in most cases.

4. That's OK, and a good one But as you said above runs via ActiveX and does install a driver. And as you said it is and on demand scanner!

5. They don't install in the first place see #3

6. If you keep the quarantine then it can be restored, if you delete the quarantine and need it you are up the creek. Same as installed online/resident Virus scanner if you clear quarantine it is gone.

Mike

Thanks to everybody who provided info.

Special thanks to mflynn for fully covering the subject, as he usually does, including point-by-point replies.

My conclusion: At my level, I better stick to one AV and only expect supplementary resident support from ThreatFire. For additional scans, Virus Cleaners do seem ideal, in view of the fact that they don’t need installation. It is most strange that I didn’t know about their existence, whereas I had read many articles about online scans, which to me appear much less attractive.

I just clarify that the BitDefender 10 Free Edition, is not the on-line scanner, but a downloadable version, which has to be installed. I understand that it is similar to the regular BitDefender, but cannot be placed on guard. It seems to me that, since a resident AV on-guard is indispensable, and this BitDefender FE cannot co-exist with it, it is practically useless.

For anybody interested, info here:
http://www.bitdefender.com/PRODUCT-14-en--BitDefender-Free-Edition.html