Updated November 2017
Let’s start from the worst case. You already suffered the attack of a virus, and you just understood the importance of being protected. In many cases a virus impedes connection to the internet. Since you read this, it is not your case; you can have reasonable hope that you will manage to repair your system in a few hours, instead of formatting your hard disk, and you will get constant solid virus protection without spending any money at all.
1. If you can not even scan your computer for viruses, because the virus has damaged your antivirus application, you can use this free and good On Line Antivirus Scanner. Scan your computer and delete any virus that might be found.
2. Download SUPERAntiSpyware and scan your computer. One of its greatest features, is that it will let you boot in Safe Mode, if you need to, even if the virus has blocked safe mode.
3. Download one of these: Kaspersky Free, AVG AntiVirus Free Edition or Avira AntiVirus Personal Edition to have real time protection against viruses, worms and trojans. However, Windows Defender, the native Windows antivirus application, is improved, I wouldn’t object if you decided to use this instead.
4. Download Malwarebytes anti-malware scanner, and use it at least once a week to make sure you are not infected by malware mainly coming from web sites. This is perhaps the best anti-malware free application. It doesn’t offer for free real time protection, but you can have AVG or Avira for real time scanning, and use Malwarebytes on demand. Combine this with AdwCleaner, a program that removes adware other scanners won’t recognise.
5. (Optional) Download Spyware Terminator to have real-time protection from known Spyware, usually offered only by commercial programs. Spyware Terminator is against spyware, adware, trojans, keyloggers, home page hijackers and other malware threats.
6. (Optional) Download Spybot-Search & Destroy. It comes in many languages, and detects and removes spyware.
7. Download a Firewall that will protect your computer from intrusions, or just keep using the native Windows Firewall, which is strong enough.
No matter what programs you will choose to protect your PC, always have a system image as recent as possible, reflecting a working state of the OS, in case you need to fight a virus by resorting to a previous point in time. And of course always backup your personal files, and make sure you avoid spam mail, sometimes leading to malicious sites.
Check also this list with the best free antivirus, antimalware and antispyware applications.