Conflicts

The primary concern with using more than one anti-virus program is due to conflicts that can arise when both are running in real-time mode simultaneously. 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 recognised 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.

Further, dual installation is not always possible because some anti-virus programs will detect the presence of others and may insist they be removed prior to installation. To avoid these problems, use only one anti-virus solution. Deciding which one to remove is your choice. Be aware that you may lose your subscription to that anti-virus program's virus definitions once you uninstall that software.

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.

In contrast, using more than one anti-spyware running in real-time mode simultaneously increases your protection coverage without causing the same kind of conflicts or affecting the stability of your system as what can occur when using more than one anti-virus. Even if your anti-spyware programs are not running in real-time, the overlap of protection from using different signature databases will aid in detection and removal of more threats when scanning your system for malware.

No single product is 100% foolproof and can detect and remove all threats at any given time. The security community is in a constant state of change as new malware infections appear. Each vendor has its own definition of what constitutes spyware and scanning your computer using different criteria will yield different results. The fact that each program has its own definition files means that some malware may be picked up by one that could be missed by another. Thus, a multi-layered defense using several anti-spyware products (including an effective firewall) to supplement your anti-virus provides the most complete protection.

As a general rule, using more than one anti-spyware program like Malwarebytes' Anti-Malware, SuperAntispyware, Spybot S&D, Ad-Aware, etc will not conflict with each other or your anti-virus if using only one of them for real time protection and others as stand-alone scanners. In fact, doing so increases your protection coverage without causing the same kind of conflicts or affecting the stability of your system that can occur when using more than one anti-virus. The overlap of protection from using different signature databases will aid in detection and removal of more threats when scanning your system for malware. However, if using all their real-time resident shields (TeaTimer, Ad-Watch, MBAM Protection Module, Spyware Terminator Shields, etc) together at the same time, there can be conflicts when each application tries to compete for resources and exclusive rights to perform an action. Additionally, competing tools may even provide redundant alerts which can be annoying and/or confusing.

However, you can over do it with resource heavy programs that will slow down you system performance. Sometimes you just have to experiment to get the right combo for your particular system as there is no universal solution that works for everyone.