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1. They will try to kill each other. Because antivirus programs search your system for programs that are monitoring and sending information about your system, a competing antivirus program that is monitoring and sending information about your system tends to look like a virus, so it will attempt to block it and remove it.
2. They will fight over viruses. When an antivirus program encounters a virus, it removes it and quarantines it. But if a competing antivirus program sees that quarantined file and wants to remove and quarantine it in accordance with its own objectives, then it will repeatedly send reports and notifications about this virus that it is detecting, even though it is no longer actually a threat to your system. If you don’t want to continually get bogus warning messages, this will be a problem for you.
3. They will sap your power. Antivirus programs use a lot of your system memory to conduct system scans and other related operations. If you have two of these operating simultaneously, your system’s effectiveness can be greatly diminished or completely wiped out altogether, and without any benefit, since the two are performing redundant operations.