Feel Free to reprint this article in newsletters and on websites, with resource box included. If you use this article, please send a brief message to let me know where it appeared: Your computer is as slow as molasses. Your mouse freezes every 15 minutes, and that Microsoft Word program just won’t seem to open.

You might have a virus.

Just what exactly is a virus? What kind is in your computer? How did it get there? How is it spreading and wreaking such havoc? And why is it bothering with your computer anyway?

Viruses are pieces of programming code that make copies of themselves, or replicate, inside your computer without asking your explicit written permission to do so. Forget getting your permission down on paper. Viruses don’t bother to seek your permission at all! Very invasive.

In comparison, there are pieces of code that might replicate inside your computer, say something your IT guy thinks you need. But the code spreads, perhaps throughout your office network, with your consent (or at least your IT guy’s consent). These types of replicating code are called agents, said Jimmy Kuo, a research fellow with McAfee AVERT, a research arm of anti-virus software-maker McAfee Inc.

In this article, though, we’re not talking about the good guys, or the agents. We’ll be talking about the bad guys, the viruses.

A long, long time ago in computer years, like five, most viruses were comprised of a similar breed. They entered your computer perhaps through an email attachment or a floppy disk (remember those?). Then they attached themselves to one of your files, say your Microsoft Word program.

When you opened your Microsoft Word program, the virus replicated and attached itself to other files. These could be other random files on your hard drive, the files furthest away from your Microsoft Word program, or other files, depending on how the virus writer wanted the virus to behave.

This virus code could contain hundreds or thousands of instructions. When it replicates it inserts those instructions, into the files it infects, said Carey Nachenberg, Chief Architect at Symantec Research Labs, an arm of anti-virus software-maker Symantec. Corp.

Because so many other types of viruses exist now, the kind just described is called a classic virus. Classic viruses still exist but they’re not quite as prevalent as they used to be. (Perhaps we could put classic viruses on the shelf with Hemingway and Dickens.)

These days, in the modern era, viruses are known to spread through vulnerabilities in web browsers, files shared over the internet, emails themselves, and computer networks.

As far as web browsers are concerned, Microsoft’s Internet Explorer takes most of the heat for spreading viruses because it’s used by more people for web surfing than any other browser.

Nevertheless, "Any web browser potentially has vulnerabilities," Nachenberg said.

For instance, let’s say you go to a website in IE you have every reason to think is safe, Nachenberg said.

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