The following is my opinion. Not all will agree. It is based on an accumulation of knowledge and experience gained in supporting ordinary Windows Computers users. This is the formula that I use for my clients that works very well. Infections are rare when they follow this advice.
1. Make sure Microsoft Windows Updates are applied as soon as available. Your computer should be set to automatically apply them. Normally Microsoft releases a whole bunch of updates (mostly security related) on the 2nd Tuesday of the month usually about 2pm EST.
2. Remove all antivirus and internet security programs.
3. Install Bitdefender Antivirus Plus and no other security product. Note the exact name of the product as I do not recommend any other. This product is an always running one and will operate quite silently and automatically until it finds a threat. In almost all cases, it will then proceed to remove the threat without your intervention. This product automatically conducts updates and scans without your intervention. Scans are not scheduled but begun only when your computer is on but not in use. If you start to use it, the scan will suspend itself and restart later when the machine is again not in use.
4. Be very judicious about giving out your email address. Give it only to people you trust. If you are receiving an extensive bunch of junk (called SPAM), the only sure way to solve the problem is to simply change your email address. Be prepared to do this every couple of years.
5. Never send or forward email to a whole bunch of people using TO: Always use BCC. Ask friends to do the same. Make sure email you send and receive do not contain any visible addresses other than yours (sender) and receiver.
6. Email attachments are the most dangerous. Do not open one, even if from a known friend unless you are expecting it.
7. Do not respond to email asking for personal information, even if it takes you to a web page that looks legitimate. Email is a postcard. Anything in an email can be viewed by others.
8. If you use a program such as Outlook, Outlook Express, Windows Mail or Windows Live Mail to download and store email on your computer, you should periodically sign on to your internet service provider’s (e.g. Rogers or Bell) web mail and check your Sent folder to ensure email is not being sent from your account that you did not send. If you find this, you need to change your password.
9. Assuming you have Adobe Flash Player, Java and Adobe Reader installed on your computer, allow them to update at any time offered.
10. Use complex passwords, do not use the same one for everything and change them periodically. Complex passwords are at least 8 characters, contain numbers (non-sequential) lower case and upper case letters and no words found in any dictionary, or names or postal codes or any recognizable form. Write down all your passwords on a piece of paper (not stored in the computer) and keep it in a safe place known by people you trust (have a pity for your executor).
11. If you use an Apple computer, It is not immune from infection contrary to what your Apple friends told you. Not only can it become infected, it can also serve as a vector to infect others. Although definitely cool, Apple computers are really no different from Windows computers except that you have the privilege of paying much more for them.
12. Windows 7 is a dramatically improved product. It is hugely more secure, easier to use and faster. If you use Windows XP, your computer will become useless after April, 2014. As of that date, Microsoft will no longer be offering security updates for Windows XP. If your current computer uses Windows XP and is more than about 3 years old, it is highly unlikely you can upgrade to Windows 7 or even if you can, it would not make economic sense.
13. Use Internet Explorer version 9 if you are using Windows 7 or Vista. Use Internet Explorer version 8 for Windows XP. IE is dramatically more secure and just as fast as the others. If IE is not fast, you can reset it. Tools, Internet Options, Advanced. Contrary to the warnings, you will not lose things when you do this.
14. Free games and porn sites are infamous for causing infections.
15. Think three times before downloading any software or updates. Read what it says in the boxes before clicking OK. Many if not most will include downloading other stuff you do not need if you do not uncheck the boxes. Toolbars abound and they are useless computer resource consumers. You do not want any toolbars. Common ones are: Ask, Bing, Yahoo, AOL, Google.
16. You can opt out of Google tracking your computer. It will stop them from sending targeted ads. Go to: http://www.google.com/ads/preferences/plugin/ to install the opt out.