Today's computer has more computing power than did the computers aboard the Gemini spacecraft. That's a lot of power, more than most people need or use. There are benign ways to use that power, as several research facilities have projects where people donate their unused computer time. However, there are also destructive ways that computers are being used, unknown by their owner.
One way, is that your computer is spying on you if certain spyware programs have been installed on your computer. What's so bad about that? It's what they are taking from you that counts. Ever enter your credit card number at a website? If a key-logger program is on your computer, they can retrace your steps and get your credit card number! Less malicious programs just track every step you take on the internet, and report it back to their masters. If this was only market surveys, it would be bad enough. However, they are also capable of doing much worse. Read on.
Note that it is no longer "cool" for a computer hacker to hurt the computer that is hacked. That is as outdated as hijacking an airliner just to make demands and take it to another city. Just as we can no longer safely assume that airplane hijackers want to live through their episode, we can not assume that hackers amuse themselves by making your life miserable by hurting your computer. They have new goals, new challenges, and a part of that is the desire to use your computer to do their dirty work. They do not want to temporarily disable, they want to place a "Trojan Horse" on your computer, so that it sits, waiting for instructions. Listening with every internet connection for commands from it's master.
Then, when it's time for actions that you would never take, both because of lack skill and lack of malicious intent, your computer (if it is online at the time) will be used as a soldier (nick-named "zombie" because it is a lifeless yet obedient servant) in a computerized attack on whatever target is chosen by the hacker. It could be a university website, it could be an on-line service provided by a bank or a government.
This step is optional, though it does show you how vulnerable your computer may be, and how it can be "enlisted" by someone else to serve their purposes.
Either go to the Gibson Research website (https://grc.com/x/ne.dll?bh0bkyd2) and check your computer. Click on "Proceed". Follow the prompts. At least do the "File Sharing" and "Common Ports" tests.
Alternatively, Windows users can go to the Microsoft website and be tested by them, instead. Go to http://www.microsoft.com/security/protect/default.asp and enter which version of Windows you have, then click "Go". Follow the prompts over the next few pages. They will analyze your computer and take you through some of the steps below.
Yes, I know you got a flu shot ten years ago, and you don't want another. The fact is that anti-virus software is nothing more than an electronic version of a wanted poster. It helps you keep yourself out of trouble by telling you (your computer) what to be on the lookout for, and whom to avoid! No "Wanted Poster" ever had pictures of "future criminals", just of current offenders. People once reviewed "Wanted Posters" to help keep safe, let your computer do the same thing by updating security software!
Install software that finds and rids your computer of spyware (software written to spy on your behaviors and report back to their master). Update it often!
When your browser opens up to a web site that you did not ask for, your homepage may have been hijacked. The best article on dealing with homepage hijacking is found at http://www.geekgirls.com/net_hijacked.htm
Consider disabling the Messenger Service on the computer. Past, present and future viruses take advantage of this feature, that no home user uses. (Control Panel -> Administrative Tools -> Services; right click on Messenger and select Properties. Set Start-up Type to Disabled and press the Stop button. Set to manual.
Be smart about passwords. At least seven characters, using letters (CAP and small), numbers and at least one symbol.
Do not open email attachments, even from friends, unless the email explicitly explains the attachment and you are already expecting it! I know this is tough, but don't even look at pictures that Aunt May is sending you, unless she sent you a message earlier telling you they are on the way. Look, you can't trust that an email is from her just because it says it is from her. These Trojan Horses can fake the return address once they have gone through your list of contacts, or they may have been sent from her computer without her even knowing about it. Not only that, Aunt May can unintentionally send you Trojan Horses. It is not safe to open any email attachment, unless you have done steps 2 through 7 above! After all, you wash your hands before you eat, right? Even if it's your own mother cooking?
If using Outlook or Outlook Express for email, keep it updated! (If that sounds like a mantra to you, use it often! Both Outlook and Outlook Express have many convenient features which automate tasks for the user, and also give opportunities to others to automate for their own purposes. You will be safer using software that does have the automation features, or has them in a different way. If the hacker doesn't know what program you are running, they don't know what commands it will follow! So, you may consider switching to:
If using a wireless network, use MAC filtering, turn off SSID broadcast and use WEP.
Periodically run Pest Patrol from http://www.pestpatrol.com/ to see what ports are being used by Trojans.
Some pages recommend you turn "System Restore" off, and you may need help doing that. So, to turn "System Restore" on or off, there's a great resource to help take you through that process at: http://vil.nai.com/vil/SystemHelpDocs/DisableSysRestore.htm