You can have more than one anti-virus program installed on your system as long as only one of them is actively running and providing real time protection. The other should only be used as an on demand scanner. However, even when one of them is not running, problems still can arise when the active anti-virus detects the non-active one's definitions or quarantined files
. Further, dual installation is not always possible because some anti-virus programs do not like others and may insist they be removed prior to installation.
The 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 "False Positives
". 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. 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.
In contrast, using more than one anti-spyware program running in real-time mode simultaneously increases your protection coverage without causing the same kind of conflicts or affecting the stability of your system.
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 layered defense using several products (including an effective firewall) to supplement your anti-virus and scanning in "safe mode" provides the most complete protection