How do you solve this issue of computer Virus?

[kinthithe]

You are toodling along, reading your email, surfing the web, updating your Facebook status when you get an alert on your screen. It says, "Your computer is infected" or something similar. The alert looks very official, like it came from your operating system. There's a button to click, so you do. Suddenly, a cool looking scan appears on your screen, showing you progress as it searches your system and finds several viruses and other malware. You think, "How can this be happening? I have security software on my computer?? Did it fail me?"