is it bad to run multiple antivirus programs?

is it bad to run multiple antivirus programs? right now I have Norton internet security 2009 and AVG antivirus on my machine. is this overkill? any negative effects to running 2 security programs.


go these programs automatically protect my external drives or do i need to do something more for them?

The best explanation as to why you should NOT have more than one antivirus is: (same thing applies to firewalls)

"Most people know they should only have one but do they know why?
Having more than one AV would catch more viruses – right?
No, WRONG! Antivirus programs clash with each other, a bit like two guys fighting over one chick.
AV programs work at a low level whereas other programs run at high level. It is ok to have multiple instances of, lets say, web browsers as they just connect to the net then close down completely when you are finished surfing and they operate on the higher level.
So what actually happens is when you have two programs working at low level is they trip over each other causing program crashes and system lockups.

Most antivirus programs operate in two modes:
Scan Mode: the program examines the memory and files on your hard disk for traces of malware. This involves examining the contents each file for traces that ‘look like’ viruses.
Monitor Mode: usually called or referred to as ‘real time monitoring’, the program runs constantly scanning files as they are downloaded to your machine, notifying you if the file you just received contains something that appears to be a virus.
It’s ok to scan with a different antivirus program than your normal resident scanner as all that happens is the program will scan, report it findings then it will close down. Online scanners are good for second opinions. But if you install a second AV program it will (or should) inform you that another antivirus program has been detected and to uninstall it, if you choose to ignore this warning it will then install its own ‘real time monitors’ and that is when the problems will start – lockups, poor performance and crashes."

(taken from pcsupport forums-Why should you never install two Antivirus programs? )

You will need to decide which antivirus program to keep and which one to uninstall.

Your external drive will be automatically protected when hooked into your computer.

The primary concern with using more than one anti-virus program is due to conflicts that can arise when they are running in real-time mode simultaneously. However, even when one of them is disabled for use as a stand-alone scanner, it can affect the other. Anti-virus software components insert themselves into the operating systems core and using more than one can cause instability, crash your computer, slow performance and waste system resources. When actively running in the background while connected to the Internet, they both may try to update their definition databases at the same time. As the programs compete for resources required to download the necessary files this often can result in sluggish system performance or unresponsive behavior.

Each anti-virus will often interpret the activity of the other as a virus and there is a greater chance of them alerting you to a "False Positive". If one finds a virus and then the other also finds the same virus, both programs will be competing over exclusive rights on dealing with that virus. Each anti-virus will attempt to remove the offending file and quarantine it. If one finds and quarantines the file before the other one does, then you encounter the problem of both wanting to scan each other's zipped or archived files and each reporting the other's quarantined contents. This can lead to a repetitive cycle of endless alerts that continually warn you that a virus has been found when that is not the case.

Anti-virus scanners use virus definitions to check for viruses and these can include a fragment of the virus code which may be recognized by other anti-virus programs as the virus itself. Because of this, most anti-virus programs encrypt their definitions so that they do not trigger a false alarm when scanned by other security programs. However, some anti-virus vendors do not encrypt their definitions and will trigger false alarms if used while another resident anti-virus program is active.

To avoid these problems, use only one anti-virus solution. Deciding which one to remove is your choice.

Most anti-virus vendors recommend that you install and run only one anti-virus program at a time:
Symantec's statement.
Avast's statement.
AVG's statement.
Dell Support advises the same for their systems.

is it bad to run multiple antivirus programs? right now I have Norton internet security 2009 and AVG antivirus on my machine. is this overkill? any negative effects to running 2 security programs.


go these programs automatically protect my external drives or do i need to do something more for them?

You never know what will happen but then again I don't think many people believe in installing more than one anti-virus program per computer. Just delete them all but one!
It takes up a lot of your ram. You only need about one antivirus program, just make it a program that runs from the time you start your computer untill you shut it down. A program like this is one that you will have to purchase.
instead of using multi anti virus and other security tools i would suggest be aware about spyware maleware and other malicious files Use any one good Anti-virus as you mentioned you are using lots of antispywares antivirus etc , using multiple AV can be result of false virus warnings and alerts conflicts and it will also consume your memory.
PC Health Advisor is exactly what you need security program, brings together high quality tools in one comprehensive program.

http://www.mywot.com/en/scorecard/buy.paretologic.com

You only need about one antivirus program, just make it a program that runs from the time you start your computer untill you shut it down. A program like this is one that you will have to purchase.

Actually, there are some antivirus programs out there that runs realtime protection that are free such as Avast, AVG, and Avira as examples.