How antivirus software works

This article arrived in my inbox yesterday & since I see people asking about it & I've wondered it myself, I hope its ok to post it in here.
Antivirus programs are powerful pieces of software that are essential on Windows computers. If you’ve ever wondered how antivirus programs detect viruses, what they’re doing on your computer, and whether you need to perform regular system scans yourself, read on.
An antivirus program is an essential part of a multi-layered security strategy – even if you’re a smart computer user, the constant stream of vulnerabilities for browsers, plug-ins, and the Windows operating system itself make antivirus protection important.
On-Access Scanning.

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