The internet is not a safe place. I hope the sentence got your attention, because it’s completely and undeniably true. Although the Internet is a place to search for information and download just about anything, you can never just assume whoever is letting you download is friendly and helpful. Sometimes, whatever you download has an invisible, but potentially problematic component.

In order to protect yourself, there are three key software pieces to defend your machine, each of which is a layer of protection on its own. By using all three together you’re increasing your operating system’s longevity, because a well-protected system is a healthy system, and runs a lesser risk of needing a complete reinstall than a completely or partially protected one. Note that you only ever need one of each layer - programs on the same layer usually don’t play together well and will interfere with each other, effectively crashing the protection layer.

Anti-virus
Anti-virus software is by far the best-known layer of protection, and which most users get with one of their computer components, usually bundled with the motherboard. There’s a whole variety of virus types - from trojan horses, which act as a legit program but do something entirely unwanted, to worms, which exploit holes in the operating and security systems, to e-mail viruses, which copy themselves onwards to people in your address book and send themselves onward like a chain-letter on steroids.

Anti-viruses generally attempt to either disinfect any infected file by removing the virus from the contents or delete the file completely. Some even have a much safer option of shredding, which wipes the infected file more thoroughly than even deletion. Generally, if the virus hasn’t been started, it’s best to destroy or shred it.

While an unleashed virus (if the virus is a destructive type) may cause total file destruction, some files may be recovered by disinfection (or healing) which removes the virus from the contents. This typically fails on any executable file such as a program, but has been known to fix documents like Word and Excel files - in case you have no backup, it might help salvage at least part of the original.

While all AV programs can work in a so-called on-demand scan mode, which means the user starts the antivirus manually and sets it to work, some offer on-access scanning, which checks files before they’re actually opened, ensuring that you don’t accidentally unleash a virus. A commonplace feature on some AV programs is called a chest or quarantine, since all infected files are usually moved there and become unopenable so they wouldn’t affect the computer - it’s best to think of the quarantine as a sandbox where the viruses are kept inactive.

To recapitulate, when deciding on your AV, these key factors should affect your
choice.

• modes of scanning - antiviruses that offer both on-demand scanning and
  on-access scanning are generally more thorough as well as more convenient,
  although on-access scanning can sometimes slow down computer performance by a
  bit
• action choice - any antivirus worht its salt will be able to delete the
  infected file. Check for the ability to disinfect (or heal), quarantine and
  shred - the more options the antivirus has, the better
• library size - obviously, the greater the library of known viruses is, the
  more viruses can be detected and eliminated
• update intervals - the closer the time between updates, the better. A week
  between updates is not good. A day between updates is good. Multiple updates per
  day are a godsend, but they come at a steep price
• critical acclaim - although biased, and sometimes fueled by commercials, it’s
  a good overall measurement because antiviruses are commonly pitted against each
  other in a stress test
• price - last but definitely not the least, how much you’ll shell out for it