According to Mark Brewer, CEO of Walnut Creek, Calif.-based Covalent Technologies Inc., almost a third of the customers it supports on the Apache Tomcat application server are running it on Windows.

"That had been 15 percent to 20 percent historically," Brewer said. Almost a fifth of Covalent's customers also run the Apache HTTP Web server on Windows, which Brewer considers equally significant, considering that Microsoft bundles a competing product, Internet Information Server, with Windows Server. More than ideology, that factor -- that Microsoft makes a huge number of business applications, a number that is only increasing -- could eventually limit the growth of open source on Windows.

"If I've already got Microsoft installed in the box, why would I bother to throw it away and install something else?" said Mike Olson, vice president of embedded technologies for Oracle and former CEO of Sleepycat Software Inc. Sleepycat, before it was acquired by Oracle in February, made an open-source embedded database that competed with Microsoft. "It's a friction-y thing to have to do," Olson said. "So as long as it doesn't suck, you're going to stick with what you've got."