The second bit of information out of the study that Zajac liked was that ESET products are popular among computer experts. To him, that's the highest product endorsement possible. "They can filter out the hyperbole of the major players in the market," he says.

And recently, he got a firsthand report from a Florida customer he helped over the phone. After answering the customer's question, Zajac asked him why he bought the ESET product. "He says, 'A friend bought it and I used it fraudulently for three months. But it was so good, I decided to buy it.' "

Microsoft Exchanges

In 2001, he got an e-mail from somebody at Microsoft wanting to know technical questions about the company's security software. They exchanged 10 e-mails or so and then Zajac says, "I didn't hear back for two weeks. I thought I had made a mistake."

Turned out the Microsoft guy's computer had been hit by the "love letter virus."

Zajac couldn't help but notice the irony. "Our product would have protected him from that," he says.

ESET's growth has earned national recognition from Inc. Magazine as among the country's fastest-growing private companies for the past three years. This year, the magazine ranked it No. 379 on its list of 500 companies.

The Business Journal ranked ESET No. 6 on its 2009 list of Fastest-Growing Private Companies for its 228 percent growth from 2006 to 2008, growing from $33.9 million in '06 to $111.4 million in revenues last year.

And even though this year's financials for the company aren't included in the Business Journal's fastest-growing list, Zajac says there is no growth slowdown for ESET, recession or not.

So far this year, company revenue is up 50 percent over last year, he says. Because the security software industry overall is growing at about 10 percent a year, that tells Zajac that ESET is taking away customers from competitors.

Heading Off New Viruses

Zajac says his software is popular with customers because it does what other security software doesn't: It heads off new computer viruses before they can attack a computer. He says that is done by creating a virtual computer to accept all messages going to the actual user computer. If the virtual computer detects unusual activity, it stores it in a memory file where it can't infect the user computer.

ESET now boasts having more than 90 million users worldwide. Growth markets are Africa, Asia Pacific, Australia, Europe, Latin America and the Middle East. Customers range from individuals to companies with more than 400,000 computers. The off-the-shelf price of using ESET security software is about $40 a month for its NOD32 Antivirus product, to around $60 a month for its more robust Smart Security software.

"It's not that cheap," says Zajac. "We don't want to give a freebie. We need money to develop and upgrade the product."

© 2009 San Diego Business Journal under contract with MarketWatch. All rights reserved.