I am writing this guide because once in a while people I know would run into problems with viruses and other kinds of digital evil so I'm writing this guide to point 'em all to this article in future, and who knows maybe it would help some others on the internet. I am very lazy so I'll keep it short and as simple as I can. Generally keeping your PC safe and online stuff safe is extremely simple if you follow some extremely simple rules.
Part 1: Keeping your PC clean from viruses.
This part is intended pretty much for Windows users only, if your on Mac or Linux you are more safe against these. If you wan't to know more how to protect your Mac or Linux system just read on how to set proper user permissions for your system.
How do this viruses get into your computer:
Most of the viruses and malware come from the internet. General rule of safety is to download stuff/open email/etc. only from trusted sources. This is especially important when free stuff is concerned. There are millions of 'free' software out there which are malware and viruses in disguise. So if you want to download software do so from a trusted source, say something from Microsoft or Apple is pretty much guaranteed to be safe. Always pay attention to the address bar of your browser when you are downloading, make sure its the right website and not a 'mikrosoft.kom' or 'apple.pie'. If software you are about to download is from a company you never heard about, research it, check if it has a wikipedia article, check if it is ranked on websites like download.com, or just put "TheAwesomeProgramYouWant is malware" in google search and see if anything will come up. By doing so you will eliminate 50% of virus threat that can come from the internet. Second virus threat from the internet comes from email attachments. You can't get a virus by just recieving it, but you can infect your computer if you open the email attachments. Every time you open an email attachment your pretty much tell your computer "I trust whatever is in there, and if will blow up my computer, so be it" <- and that's true about pretty much any other program you open your computer, most often you give it same level of control over your computer as you have... I got carried away, so email attachments - triple check from who the email came from, do you absolutely trust the sender? Generally text documents, images, spreadsheats are less likely to harm your computer, but you have to be able to distinguish between a program attachment and say image attachment before opening it. The good news is that most of the modern public email providers such as yahoo, gmail, gmx have built in virus scanners that scan the emails before you get them, but you can never be sure. So again, its all about trusting the sender of the email. Email viruses are I guess about 30%... or whateva% of virus threat for average internet user. The less common but yet very dangerous way of getting your computer sick is getting it infected from a flash drive or a CD that has been on someone else's computer. If the cd/usb flahs drive/memmory card is not your's, someone else has written on them than its better not to put em into your computer. Pictures/videos/audio are generally safe to copy from these, but the problem is that unlike with email attachments the virus can be sort of invisible for an unnarmed eye, they can be invisible, hidden, or affect your windows system before anyone knows it. I would probably be OK with a memmory card from friend's photo camera, but I would double check their flash drive with an antivirus before opening it. That's anothether 10% of virus threats, and the last but not least are the viruses that spread through the local network, such as your home network or your office network, not the internet. The only good way to protect against these is to have your system firewall running. Just google "windows XYZ firewall" and learn how to check if it's on.
How to defend against viruses/malware/spyware etc.
1) Have your Windows updated! I can't stress enough how important is this. Always do windows updates whenever it asks you. Oh and if your are still on Windows XP or older then just forget about it... I know Windows XP is still corporate standard (as of December 2011) at many companies but really its just too old already. Move on to Windows 7.
2) Have your Internet browser updated! Ok so unlike Windows which is preconfigured to run updates automatically your main favourite internet browser might not be, so check once in a while if your are running the latest version.
3) Have your every-day frequently running software updates. Such as office programs, email-client, instant messanger etc.
4) Have a free decent antivirus program installed on your computer. Why free? well because since you are reading this article you are probably someone I care about so, I wouldn't want you to waste your money on something useless that just makes your computer run 2 times slower. As of today my recomendations are: Microsoft Security Essentials or Avast! There are plenty of other good ones but these two happend to be my favourites.
5) Have an emergency malware wiper instelled on your computer. Ok so antivirus protects your computer by constantly scanning it and tracking all programs you are using in real time, and surprise - it misses sometimes. So your computer gets infected and your antivirus is unaware. There are other sorts of protection software that specialize in quick scanning and recovering your system in on-demand fashion. Now go get Malwarebytes. The free version is all you need, just run whenver your computer is in trouble or you suspect it's been infected.