Inspired by recent virus-infestation of one of my family member's computer I've decided to write a small guide to computer self-defense for beginners - that is, to write down things that are obvious for every computer scientist, but not for every user. I emphasize I'm not a security specialist so for some of you my thoughts may look lame :)
Historical background: Many years ago people used to copy programs and transfer them between computers using floppy disks. Viruses were infecting EXE files by adding their code to programs and once someone ran such file, the virus spread to other programs in his computer. Viruses were sometimes written for fun. Some of them were harmless, while other did "pranks" such as formatting whole hard drive :) Now we live in the Internet era and programs downloaded from the Web or from CD/DVD disks require installation, so viruses like these are no longer effective.
Today's viruses are no longer infecting particular EXE files. They are rather kind of worms or trojan horses (however you call or classify them) - they just install themselves in Windows and run in the background as separate programs. They are neither destroying whole computer nor totally harmless. Modern malware software is part or organized crime, so it can, for example, steal user's passwords to online bank account, MMORPG game or use his computer to perform DDOS attacks and sending spam.
So how (not) to catch a virus? Viruses are not usually able to infect a computer without user's initiative. It's quite safe to use Windows, be connected to the Internet, browse any websites, read any emails, open any files such as music (like MP3), images (like JPG), archives (like ZIP) or documents (like PDF). The exception is when one of the programs have a bug which can be exploited to execute code embedded into document's data. Such critical security bugs are fixed quickly so the security procedure for them is to update your software regularly - use Windows Update / Microsoft Update and install new versions of programs you use, especially these connecting to the Internet.
The only possibility when a virus can run without user's approval is Autoplay technology for flash drives and CD/DVD disks. Each time you enter such media, Windows looks for autorun.inf file and can automatically run prepared program. As pendrives are very popular nowadays, I consider critical for computer security to turn off the Autoplay functionality. You can do it using free Tweak UI tool, just like that:
It's amazing to me how some people constantly suffer from viruses on their computers while other almost never catch any. Do the second never visit porn sites or use cracks and pirated software? Maybe, but I think it's rather the matter of obeying some simple security rules. Websites, emails or image/music/document files are not able to run arbitrary code on your computer. You must explicitly agree for that. This danger comes in two forms. First one is when a website wants to run/install something on your computer, usually using ActiveX technology. You can see warning about that asking if you really want to allow the website to run such program. Virus installs itself on your computer only if you agree to that.
The second danger is when you just run new EXE file. How to distinguish between safe and dangerous executables? First of all, never run unknown EXE files just for fun. Never trust it's a new brilliant porn screensaver, good and free antivirus software or a document in executable form (like image gallery, video, ebook, archive), even if you have it from your best friend. Pirated software and cracks/keygens from P2P networks also very often contain viruses. On the other hand, you can be almost sure it's safe when you download a well-known application from its author website or a website such as SourceForge.net. If you are not sure about an executable file and you really need to run it, scan it with an antivirus first.
I believe these simple rules are worth much more than not using Windows, Internet Explorer, Outlook Express or cracks/pirated software and using any firewall or resident antivirus protection.