The Basics about Viruses

A virus is a small program that spreads from one computer to another computer, like a microbe.
It's like a cold, infecting your whole computer; although it may not be immediately obvious that your computer has been inhabited by a virus, you can tell when certain symptoms arise.
• Your computer behaves strangely, i.e. in a way that you haven’t seen
before.
• You see unexpected messages or images.
• You hear unexpected sounds, played at random.
• Programs start unexpectedly.
• Your personal firewall tells you that an application has tried to connect to
the Internet (and it’s not a program that you ran).
• Your friends tell you that they have received e-mail messages from your
address and you haven’t sent them anything.
• Your computer ‘freezes’ frequently, or programs start running slowly.
• You get lots of system error messages.
• The operating system will not load when you start your computer.
• You notice that files or folders have been deleted or changed.
• You notice hard disk access (shown by one of the small flashing lights)
when you’re not aware of any programs running.
• Your web browser behaves erratically, e.g. you can’t close a browser
window.
A person may spread his cold to another when in close contact.
Likewise, once the virus gets into your computer system through a file attachment or link in your email.
Viruses may delete or corrupt your files, among other things.