13th anniversary of the Chernobyl nuclear disaster in Ukraine.

As hundreds of computer viruses appear each month, anti-virus firms stay busy. The threat of a computer meltdown by the looming "Chernobyl" virus has many people rushing to secure their PCs, and they are turning to a handful of companies that keep a watch on the estimated 300 new viruses created every month. The Chernobyl, or "CIH," virus, which comes on the heels of the Melissa e-mail virus about a month ago, threatens to wreak techno-havoc by destroying files on a computer_s hard drive. April 26 is the 13th anniversary of the Chernobyl nuclear disaster in Ukraine. Variants of the virus already have been detected in Asia and elsewhere, where in some cases the virus is activated on the 26th of every month. Most personal computers already have anti-virus programs installed. But users are being warned that they need to update their software with new programs to ward off the new strains. To get this software, users are directed to the Web sites of a number of companies, mainly Symantec Corp. (SYMC), Network Associates Inc. (NETA), maker of the Norton anti-virus that is installed on many PCs, and Trend Micro. When these headline-making scares hit, anti-virus companies see more business, but they don_t necessarily get a huge boost, analysts said, since about 75 percent of personal computers on the market already have anti-virus equipment installed. Anti-virus software accounts for at least $700 million in revenue each year, estimates Richard W. Davis, of Richard W. Davis & Co. And whenever there is a hyped-up virus such as Chernobyl, Melissa, or Michelangelo -- a virus that struck several years ago -- people become more cognizant of the problem, he said. "I think it makes people aware that there are people out there that like to wreak havoc," he said. "That makes a good basic market for anti-virus products."