It's essential for some jobs, handy for most, but don't be fooled—the personal computer can be a job ender. Even as you read this story, you should probably be asking yourself: Am I actually allowed to browse online and read news stories at the office?

The parameters for computer use at work (and even at home) are often confusing. We communicate, network, watch our TV shows, do our grocery shopping, and get our news on our computers. But it's no free-for-all. Employees should know exactly what their employer's policies are for E-mail and Internet usage, because workers are losing their jobs after computer-based missteps. Here are five ways to log on and lose your job:

Blog it up. Blogger Chez Pazienza worked as a producer at CNN's American Morning until mid-February, when Pazienza says his boss informed him that the company discovered his name attached to blog posts written without CNN's approval. Pazienza was fired soon after. Pazienza runs Deus Ex Malcontent, where he writes about Oprah and President Bush with equal abandon. He hadn't identified himself as a CNN employee on the blog, but CNN spokeswoman Barbara Levin says company policy is that employees must first get permission to write for a non-CNN outlet. Levin didn't elaborate, noting that the company does not comment on personnel matters.

There's even a term for being fired because of a blog—it's called being "dooced." While some blogging advocates say a well-executed blog can boost your career by presenting your best side to the HR executives Googling you, there are limitations. Dooce.com founder Heather Armstrong writes on her site that she lost her job a year after beginning the blog for writing entries that involved colleagues. She now tells site visitors, "Be ye not so stupid" and offers parameters for safe blogging: "Never write about work on the Internet unless your boss knows and sanctions the fact that you are writing about work on the Internet."

Play a way. Solitaire, that ever seductive way to while away the hours, is probably not a great choice for the workplace. New York City Mayor Michael Bloomberg caught sight of a solitaire game on a city employee's computer screen in 2006 and fired him. "I expect all city workers, including myself, to work hard," Bloomberg said then.

Richard Bayer, an economist and chief operating officer of the Five O'Clock Club, an outplacement and career coaching organization, says employees who use a company computer for personal matters on company time—whether playing solitaire or checking on their 401(k)'s—are essentially stealing from their employer. "It's a new, 21st-century form of theft," Bayer says, adding that a couple of personal E-mails each day are within reason.

Look at pics. Yes, those kind of pics. Think about this: Nearly one third of bosses have fired workers for misusing the Internet, according to a recent study by the American Management Association and the ePolicy Institute. Eighty-four percent of those employers said the reason was the viewing, downloading, or uploading of inappropriate or offensive content. The Washington, D.C., city government recommended the termination of nine employees in January for allegedly looking at pornography sites at work. The key here is privacy—as in, tell yourself you have none. The computer system belongs to the company, and courts have consistently sided with employers when it comes to computer-related terminations, says Nancy Flynn, executive director of the ePolicy Institute.

Post your pics. Social networking may quickly gain an air of formality. The mayor of tiny Arlington, Ore. (population nearly 500), grabbed headlines recently after she was recalled by voters. Among other issues, the residents were sharply divided over the propriety of photos of the mayor dressed in her underwear that were posted to her MySpace page.

Employers are beginning to monitor social networking sites, Flynn says. Not only do companies fear employees posting proprietary information, but they also don't want to find photos of the boss dancing on the table drunk at the holiday party. Opinions posted that run contrary to company values can also get employees into trouble.

Write R-rated E-mails. More than a quarter of employers have sent an employee packing for E-mail-related offenses, according to the American Management Association/ePolicy Institute survey, and 62 percent of those said it was for inappropriate or offensive language. When you write, just assume that someone inside the company is reading it. Most of the 43 percent of companies that monitor E-mail do it automatically, but 40 percent have live human beings reading and reviewing it.