This might polarise the audience a bit but I just wanted to expand on my thoughts in this earlier article.
In that article, I noted the only 100% effective solution to cure an infected computer is to format and reinstall or restore from a known good backup. Today, I tried and tried unsuccessfully to eradicate a trojan from a computer belonging to a business which drove their primary point of sale terminal. Unfortunately, it was so deeply rooted that it could not be removed manually or with specialised tools. Fortunately, they had another computer on hand to swap in to keep things going.
The problem is that antivirus software won’t catch 100% of the bad stuff coming in with pattern-based current technology. In order to detect a virus, it first must be known but there will always be a window of time where new viruses, worms, trojans and the like where they can get in and your AV might let it slip through. This is where being cautious with your online activities and portable storage comes to the fore.
Do you trust the site you are browsing or from which you are downloading a file?
Do you know where that storage device has come from and the computers to which it has been connected?
Were you expecting an attachment from that particular person?
If the answer to these questions is no, then you might want to think twice. These are popular vectors for malware infection.
Prevention is always better than the cure particularly as the cure cannot be known to completely reverse what has been installed. Also, regular and clean backups are also going to save you from losing data and time reinstalling and reconfiguring everything from scratch after an incident.