Updated December 2013
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, go to BitDefender, a free and good online 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.
4. Download Malwarebytes anti-malware scanner, and use it regularly to make sure you are not infected by malware mainly coming from web sites.
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.
In most cases, especially if you are very careful when you use your computer, steps marked as optional can be skipped. A good firewall and antivirus can be enough, with the special help of Malwarebytes, SUPERAntiSpyware. AVG Antivirus with PrivateFirewall is an efficient combination; they also seem to cooperate well, and they will not slow down your PC. With the (otherwise very good) Avira antivirus and especially with Comodo firewall I faced some problems.