How Much Do Computer Virus Attacks Really Cost?

An Anonymous Coward asks: "I'm presently doing a research project on the actual cost of computer viruses to companies within the U.S. Computer Economics, a research firm out of Carlsbad, California, has released statistics suggesting that virus attacks have cost U.S. businesses $17.1 Billion in 2000. That figure has gone on to be quoted in a number of other publications such as an article in Information Week magazine, but beyond a simple explanation, statistics aren't presented to back up this claim. How much have virus attacks cost you or your company?" To be honest with you, I too would like to see the mathematics behind this claim.