I want to know if I'm using two different  Antiviruses at the same time on my computer, will it have any affect on my system? Will this give me double protection or is there any downside to this? For example, Is the free Microsoft Antivirus combined with any of the ones in the market?
Running two antivirus simultaneously is unnecessary. Each program on our computer utilizes RAM and can possibly slow down other active programs. Two antivirus programs may conflict with each other. When you download a file, both your antivirus software will attempt to scan it. The result will be, other programs will fail and your computer will become unstable. Other antivirus can cause false result. It will notify you that there is a virus on your computer when in fact, it doesn't have one. If one antivirus is not enough for you, you can just extend it with some popular malware scanner or anti-spyware.
Antivirus are used for scanning and monitoring. And for some, they say there is nothing wrong with running two antivirus if you will just use it to scan. There will be no conflicts. However, monitoring is another story. It can cause crashes of your antivirus software, display different error messages or show different types of failure. So it is not recommended that you use two antivirus at once. To make it short, it can cause conflicts or compatibility issues, false virus warnings and less protection.
