MSN TV users are inadvertently calling emergency services after falling prey to a prank program that changes the daily dialup number on their set-top boxes to 911.

The program arrives in an e-mail message with the subject line "NEAT" and has been plaguing users since at least April, according to posts on newsgroups regarding WebTV, the former moniker for Microsoft's interactive TV service.

"When my mother tried to log on to WebTV, it started to dial 911," said one woman in a newsgroup post. "She shut it off but we got a call from the police department anyway checking to see if anything was wrong."

"One lady in her group ended up with the sheriff knocking at her door," the woman wrote.

The prank is not the first time that a malicious program has been used to call 911. In April 2000, the National Infrastructure Protection Center warned that a computer virus was causing infected PCs to dial emergency services.

Many users have called the MSN TV program a virus, and it's not readily clear if it can spread on its own. One site claims that it will send itself out to others using the MSN TV box's e-mail system, but the report is unconfirmed.

A Microsoft Network spokesperson said the program has not had a wide effect on the company's customers. "Apparently it was an isolated incident and it affected only a handful of customers," the representative said.

The representative said a patch will be issued later today in response to the 911 attachment. "This should take care of the issue, so other MSN TV customers won't have to worry about it," the representative said.

Many details of the program are still unclear, but according to a MSN TV-related Web site known as Diane's News Source, the prank code also changes fonts and other MSN TV settings. The messages carrying the prank program can come from many sources, including people that you seem to know and sometimes arrive with subject lines other than "NEAT."