Does your hard-drive or network-activity light flicker for no apparent reason? While there may be a legitimate reason for it, this could also be a sign that a virus or a hacker’s back-door program (a devious little program that allows secret access without your permission) is running on your computer. You might be donating some of your computer resources to a hacker and be largely unaware of it. Here are some examples of what could be going on if a hacker has gotten control of your computer:

1. The hacker could be using your computer to send thousands, even millions, of those annoying spam messages to people all over the Internet.

2. The hacker could be using your computer to launch attacks on corporate computing networks. In a DDoS (distributed denial of service) attack, for example, a hacker instructs thousands of “zombie” computers (like yours, perhaps) to send lots of messages to a particular corporate Web site, glutting its communications and knocking it off the Internet.

3. The hacker could be using your computer to scan other networks, hunting for vulnerable ports (communication channels for particular computer processes) that can mean more potential-victim computers.

4. The hacker may have installed spyware that reports back to the bad guys without the victim’s (your) knowledge. One example is a key logger — a small program that records every key press and mouse movement in an attempt to learn your bank-account numbers, credit-card numbers, and other sensitive information that you probably don’t want strangers to know about. (For more about this insidious stuff, see “Blocking spyware,”