"The problem has shown that many people just don't know how to use the Internet safely," said Takeshi Sato of the government's National Information Security Center.

File-sharing programs like Winny are used to find and get files — from music to video to documents — from the computers of other people also using the software. The PC owner typically has control over what is made available by limiting sharing to a specific folder.

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The virus takes advantage of this culture to propagate itself by playing a "social" trick on users, said Telecom ISAC Japan's Saito.

When the virus is activated on a computer, it first chooses a new name for itself by taking the names of other files users are likely to be searching for — usually photos or music. The resulting new name becomes so long that, under normal Windows' settings, the three-letter file extension that indicates the type of file disappears from view, he said.

Careless users who download the file will see only the name and think it is something they wanted — say, a photo of a favorite movie star. They don't see that they are actually trying to open an application, not a picture.

When they do, the virus then looks on the computer for the Winny application, grabs random files off the hard drive and uses Winny to make those files — and itself — available for download on the network.

And so the cycle repeats.

Virus variations multiplying
New strains of Antinny appear all the time. Software maker Trend Micro listed 46 variations of the virus in its database as of mid-May. Trend itself lost sales data due to a Winny leak in 2005.

"Just keeping your antivirus software up to date isn't enough, because the updates can't keep up with all the new strains of the virus," the government's Sato said.

The government's concerns about Winny go beyond viruses. It's often used to share files — and that often means illegally exchanging copyrighted materials.

Winny was already on the government's radar screen in November 2004, when its creator — then an instructor at the prestigious University of Tokyo — was handed a three-year suspended sentence on charges of violating copyright laws.

But now it is confidential data rather than hit songs that have Winny back in the spotlight.

Japan Airlines, for example, discovered last December that an Antinny-infected computer owned by one of its co-pilots leaked passwords for restricted areas at 16 airports around Japan as well as Guam's international airport. The airline was forced to alert the airports to have passwords changed as a precaution.

In early March, Japan's National Defense Agency said it lost "confidential information" due to a Winny leak, again from an employee's home computer. While defense officials refused to say what data had been lost, a news report said it included reports on training exercises conducted in Okinawa with U.S. troops in 2005.

CONTINUED : Virus attacking other file-sharing applications...

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