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