Tomorrow, many Microsoft Windows users may discover that a difficult-to-eradicate computer virus has overwritten many of their computer files with zeros.

Yale’s Information Technology Services said the virus, known as the W32.Klez Worm or “Klez,” has infected a large number of computers on the Yale campus and activates on the 6th of every month.

The virus also disables older versions of virus protection software such as Norton AntiVirus and has the ability to spread a sister virus over file-sharing systems.

Microsoft Outlook users are particularly vulnerable to the virus, which disguises itself as an attachment from real or official sounding e-mail addresses like Merely viewing an infected e-mail on Outlook will infect a computer.

“This worm is quite difficult to remove from a system,” the ITS Web site warns users. “If your computer becomes infected, unplug the network cable to prevent the virus from spreading and contact your computer support provider for assistance.”

Computing assistant Andrew Petro ’04 said it has been difficult to remove the virus from computers if users had not already installed an updated version of anti-virus software. Although computing assistants have had success in some cases removing the virus after waging a large campaign against it this weekend, Petro estimates that around 30 cases of the virus remain.

“It’s really been a pain,” Petro said. “We’re up against a deadline. It’s been an experience in the trenches.”

In situations where computing assistants fail to remove the virus, Petro recommends that users unplug their computers for the day tomorrow because data is not recoverable after contaminated by the virus.

“In terms of preventing it from zapping computer files, you should unplug your computer for the 6th,” Petro said. “That should prevent it. In terms of a permanent solution I wish I knew one.”

Petro also recommended that in general students should back up their hard drives frequently to minimize the damage any virus can cause.

Outlook users can avoid acquiring the virus by downloading a patch and by updating their anti-virus software.

Loriann Higashi, the coordinator of Academic Media and Technology, said that at least about 50 computers on campus were infected at the virus’ peak. She added that e-mails from residential college master’s offices had made students aware of the virus.

“I suspect that because of the e-mails that were sent out, most students were aware of the viruses, and students were concerned and tried to learn for sure to see if they were infected,” Higashi said.