Infected By A Virus?
You are searching around the internet one day when a popup windows tells you your computer is infected. "I have antivirus software", you say. Don't believe everything that pops up. It's usually trying to sell you something. The time to be concerned is when your computer seems to have a mind of it's own. For example, you click a link to your favorite website and the browser takes you someplace else. You try to open a file and nothing happens.
When your computer acts like this, its time have a virus scan to find and remove the threats. Most malware and virus programs come in from email and internet popups, you "allow" the infection by clicking the button or link.