NEW YORK (AP) _ The e-mail message from your colleague is a welcome diversion: ``Check out this cool site,'' he writes, giving you an address on the World Wide Web. You click the mouse, and read ...

``Shall we unleash the virus now?''

Happy Halloween. The ominous message is a trick from PC World Online, the Internet version of PC World magazine. Editors say the holiday hoax is offered in good fun, but computer and ethics experts fear the joke is just too ghoulish for today's workplace.

``It's like yelling 'fire' in a crowded theater,'' said W. Michael Hoffman, executive director of the Center for Business Ethics at Bentley College in Waltham, Mass. ``It's a bad practical joke.''

The virus warning, along with a program that appears to delete files, another that says ``I've got your credit-card number here,'' and a host of error messages, is all part of a hoax created by the magazine's monthly columnist Gil Bates.

Get it? Gil Bates? Like Bill Gates?

The offerings range from the truly harmless, like Halloween-themed screen savers, to the alarming, like an authentic-looking ``Windows Meltdown in Progress'' warning.

``Most people will get a charge out of it,'' said Bates, whose real name is Matthew Lake. A similar feature for April Fool's Day garnered scores of complimentary e-mail messages, with only a few critics, said executive editor Yael Li-Ron.

The pranks are not uncommon in today's plugged-in society. While Bates' feature works for PCs, similar versions exist for the Macintosh. They're generally created by rogues with good programming skills and an urge to stir things up, not wreak havoc, said Bill Orvis, a security specialist with the Computer Incident Advisory Capability.

But while the pranks may tickle the techno-savvy, they can confuse and upset others.

Andrew Merkle, a computer consultant in Ithaca, N.Y., was once called to Cornell University when a secretary couldn't escape from a ``system error'' message accompanied by the dreaded Macintosh bomb icon. Every time she tried to click in the ``restart'' box, the box jumped to another place on the screen.

``She was in tears,'' Merkle said. The solution _ clicking on the bomb icon _ didn't occur to anyone in the office. It took time, and ``more than $100'' billed to Merkle, to end the joke.

Joe Duraes, a graphic designer in Manhattan, checked his e-mail one day last week and found a friend's suggestion to check out PC World's Web site. He typed in the address, and a box popped up on his screen informing him that all his files were being deleted.

As the names of the files flashed before his eyes, ``I freaked,'' he said. ``I mean, I'm a designer. If my hard drive dies, my career dies.''

Duraes hit the keyboard frantically for about 45 seconds. ``Then somehow,'' he said, ``it occurred to me that it couldn't be real.''

Sure enough, there's a disclaimer at the top of the screen that hints strongly that it's a joke, followed by a link back to the magazine, which explains the whole ruse.

But Laura Pincus Hartman, director of the Institute for Business and Professional Ethics at DePaul University in Chicago, said a panicked employee isn't likely to notice fine print. By the time the joke is over, nerves can be rattled, a workplace disturbed.

During a genuine computer shutdown or virus scare, she pointed out, ``Everything stops. There's a huge impact, and it's really disruptive.''

Orvis, whose agency investigates computer security for the U.S. Department of Energy, points out that any outside program has the potential to introduce a real virus. ``If you are going to download these things, you really have to scan them for viruses,'' he said.

But in some workplaces, he said, they can be a big hit.

``Whether they're appropriate depends on the person you're doing it to and the type of workplace you're in,'' he said.

Orvis has even been duped by his school-age children, who have given him programs that skewed his screen and warped his type. And once Dureas calmed down, he liked the prank so much he forwarded it to friends, co-workers, even his boss.

``I wouldn't have sent it to everyone I know,'' he said, ``but I sent it to the people who I thought would get a kick out of it.''

Even the perpetrators, while unabashedly proud of the pranks, know there's a time and place for them.

``I have to hope that people won't be sneaking it onto the CEO's computer,'' said Bates.

``It's a juvenile prank,'' acknowledged Li-Ron. ``But hey, it's Halloween.''