What are the signs that your computer is under attack, or being used to attack another computer? Here are some behaviors to look for in determining if your computer has been attacked:
An unusual amount of hard disk activity — especially when your PC isn't being used.
An unusual amount of modem use — especially when you're not browsing the web or using email.
An unusual number of returned email messages appearing in your inbox.
Email messages appearing in your “Sent” folder that you did not send.
If you connect to the Internet via a dial-up connection, your PC automatically dialing into the Internet, without your prompting.
Missing or edited files on your hard disk.