Damage from the MyDoom virus may exceed $4 billion worldwide, making it one of the most costly internet attacks, according to Computer Economics, a research firm based in Carlsbad, Calif.

In terms of economic impact, MyDoom ranks second, below the Love Letter virus, which caused about $7.75 billion in damages, said Mark McManus, the firm's vice president of technology and research.

Network Associates estimates that 500,000 PCs were infected by MyDoom worldwide.

www.computereconomics.com, www.nai.com