What are the symptoms of an infected computer?

It’s not always easy to tell if your computer has been compromised. More than ever before, the authors of viruses, worms, Trojans and spyware are going to great lengths to hide their code and conceal what their programs are doing on an infected computer. That’s why it’s essential to follow the advice given in this guide: in particular, install Internet security software, make sure you apply security patches to your operating system and applications and backup your data regularly.

It’s very difficult to provide a list of characteristic symptoms of a compromised computer because the same symptoms can also be caused by hardware and/or software problems. Here are just a few examples:

• 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.
  
   http://www.kaspersky.com/symptoms