Yes, it is okay to have multiple programs but they could “beat up” on each other and reduce the performance of the machine. The best option is to have the best of breed in each area or have the program suite from a single vendor who should have tested the applications working together.
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Definitely not, it is not advisable to install multiple or more that 1 security software in your computer system. Remember that every antivirus software constantly scans the entire computer system for potential threats and this could really make your computer in a very slow condition. In addition, it would make your entire system crash because of the conflict between antimalware programs installed.
Multiple anti-malware programs won’t necessarily cause problems on your machine unless you have more than one configured to scan in real time for threats. If you turn off this feature in all but one application, you should be OK. Then you can manually run individual scans with each tool to thoroughly clean your machine. I have done this in the past with little or no performance degradation. As Michael mentions, a suite with multiple malware tools designed to run in concert with each other is a good solution. Microsoft ForeFront Endpoint Protection is a good one, as is Symantec Endpoint Security.