To elaborate a bit more. 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, as jgweed pointed out, even when one is not running, you have to be careful because the other may detect the other's definitions (or quarantined files).
The concern with using more than one anti-virus program is due to conflicts that can arise from them both running together at the same time in real-time protection mode. Anti-virus software componets 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 defintion databases are the same time. As the programs compete for resources required to download the necessary files this often can result in sluggish system performance.
While operating in real-time mode, each program will often interpret the activity of the other as a virus and there is a greater chance of them alerting you to "False Positives". Further, if one Av finds a virus and then the other also finds the same virus, then both programs will be competing over exclusive rights on dealing with that virus. Each piece of AV software will attempt to seize the offending file and quarantine it. If one AV finds and quarantines the file before the other one does, then you encounter the problem of both AV's wanting to scan each other's zipped or archived files and each reporting the other's quarantined contents. This can lead to a repetivite cycle of endless alerts that continually warn you that a virus has been found.
Symantec strongly recommends that you run only one antivirus program at a time. See: http://service1.symantec.com/SUPPORT/nav.n...000031316555206
Edited by quietman7, 14 June 2006 - 07:51 AM.