The "butterfly effect" gained a lot of popularity with the release of the less-than-Oscar-worthy Ashton Kutcher movie of the same name. But the concept has been around for a lot longer -- since the 1960s, as a matter of fact, when a meteorologist postulated that a butterfly flapping its wings in Brazil could cause a ripple in weather patterns that generates a tornado in Texas.

As fellow Fool Rich Smith has recounted, Microsoft (NASDAQ:MSFT) created its own butterfly effect in the antivirus software industry yesterday, when it announced that it was buying tiny, pre-IPO Sybari Software for an undisclosed amount. It was the tech giant's second such purchase in three months -- it bought up Giant Company Software in December -- and signified that Microsoft was intent on moving into the antivirus industry.

Microsoft's wing flap sent ripples -- and shivers -- across the industry and sent the stock prices of antivirus makers McAfee (NYSE:MFE) and Symantec (NASDAQ:SYMC) down 6% and 8%, respectively. Online e-commerce provider Digital River (NASDAQ:DRIV) also got caught up in the ensuing tornado. With Symantec being its largest customer, accounting for 25% of its revenues, Digital River saw its share price knocked down by 15%, falling by more than $5 a share.

Computer security is a growing concern, particularly with Microsoft products. At the same time it announced buying Sybari, Microsoft also released a security warning about eight new "critical" flaws in its Windows and Office products that could give access to hackers. (It also identified four lesser threats.) In January, it began offering free spyware-blocker downloads to prevent programs from generating unwanted pop-ups and secretly recording user activity.

While Symantec does account for a substantial portion of Digital River's revenues, the sell-off in its shares seems a bit overblown. Sybari is in the enterprise market, selling antivirus and anti-spam software to more than 10,000 businesses and government agencies. It's not in the PC business, though one imagines that Microsoft might be able to convert it to such use in the future. This is more akin to Symantec's Brightmail, which it purchased last June, and which would seemingly have little impact on Digital River.

Equally important, the e-commerce provider has joined forces with Microsoft to offer digital downloads to more than 200 online retailers, including Best Buy (NYSE:BBY), OfficeMax, and Staples (NASDAQ:SPLS). Digital River also has a huge roster of companies -- some 40,000 -- for which it builds and manages their online business. Even if Microsoft was able to make inroads into the business that Symantec supplies -- and there's no guarantee of that -- the revenues applicable to Digital River would not necessarily be substantial.

Last month, shares of Digital River sold off following news that Computer Associates (NYSE:CA) was offering free trials of its firewall and security software, another perceived threat to Symantec. Last year, it was a lost contract with Major League Baseball. This month, it's Microsoft's buying an antivirus maker. Through it all, Digital River has reported record earnings and continued growth, and the latest brouhaha seems to be nothing more than traders making bets rather than investments.

It looks to this Fool as though it's more of a butterfly caught in a breeze instead of a gale.

Read more about the digital-download revolution:

Fool contributor Rich Duprey enjoys canoeing on the Lamoille River in Vermont. He owns shares in Digital River but does not own any of the other companies mentioned in the article.