Your happily surfing the net when suddenly your browser switches you to some strange site, a pop up spawns on your screen and suddenly you are being alerted that your computer is infected.
Your system has just been compromised by some malicious software, or malware.
Malware are programs distributed by companies that get downloaded and installed to your computer, usually without your knowledge.
Malware can be attached to other seemingly innocent programs like Weatherbug, screen savers or a simple flash game.
They often bypass your antivirus because:
a) You installed the program and you are an Administrator on your computer;
b) You overrode your antivirus program. When your antivirus program popped up saying, “are you sure you know what you are doing? Do you really need this [cute screensaver / greeting card making program / free game]?” you answered, “you betcha!”;
c) The malware / virus is clever and so new your antivirus program hasn’t got a clue what it is…
They can also be embedded right into the website you’re visiting, so that simply looking at their page infects your computer. If your computer’s problem is that it’s running slow or spawning popups or redirecting you to another website at random, this is most likely the cause.
Although this video is slightly dated, it is still very much relevant and true in today’s world of online computing:
How do you clean your system? That is an article for another day. But you may find some clues in our Basic Computer Maintenance Course Notes.
In the meantime, don’t install unnecessary software, especially freebies of questionable origin, keep your antivirus up-to-date, your OS up-to-date, your Java up-to-date, and use a safe browser like FireFox w/ AdBlock Plus enabled.