Although the virus has yet to do anything more than pester its victims, Ms April Goostree, an antivirus researcher at security company McAfee.com, said the attachment could bring down a mail system if enough programs are run and are sending out response e-mail to all the addresses within the system.

Santa Clara, California-based McAfee.com has seen at least 100 samples over the past four days, including some from at least 10 Fortune 500 companies, Ms Goostree said. Many of the e-mail are written mostly in Spanish, leading Ms Goostree to believe that the virus may have come from South America. In arrangement with India.CNET.com McAfee.com declined to say which companies have been hit by the virus, although The Associated Press, citing an unidentified source, reported that petroleum company ExxonMobil and chipmaker Intel have been affected. The virus places an icon on the computer's desktop that looks like a blue eye, Ms Goostree said. "Although we haven't seen any computers being damaged, there is the potential for it to cause your Windows system to lock up" if not remedied early on, she said.

The program can be closed manually, and several antivirus companies say they have developed software that both removes and protects against this type of virus. Several viruses have taken on holiday themes in the past. Last year, a rapidly spreading computer virus built to reformat computer hard drives on Christmas Day was detected by experts after it had already cropped up on three continents. While the Navidad virus has not caused severe damage to computers, McAfee.com is still warning its customers because the virus is spreading fast among large corporations, hitting companies based in England in the past two days, Ms Goostree said. McAfee.com suggests that consumers install an antivirus program and keep it updated at least once a month.

The company also recommends using an antivirus scanner to check an e-mail attachment before clicking on it.