The most likely sign that your PC has been infected with a computer virus is that your computer starts acting differently. You might experience one of the behaviors below:
Strange messages or graphics are displayed on your computer screen.
Normally well-behaved programs are acting erratically.
Certain files have disappeared from your hard disk or your system is acting sluggish.
Your system is unable to start.
Your friends are receiving emails with suspicious files attached from your email address (that you never sent).
If your computer exhibits one or more of these symptoms, especially if you have just downloaded a file from the Internet or received a suspicious email message, your computer may be infected with a virus.