Microsoft's Windows edged out Unix in 2005 as the best-selling server operating system, research firm IDC said Wednesday, marking the first time Unix hasn't held the top spot in more than a decade.

Windows servers accounted for $17.7 billion in revenues last year, the Framingham, Mass.-based research company said, while Unix-powered servers took in $17.5 billion.

Linux, the open-source offshoot of Unix, accounted for $5.7 billion in server sales during 2005.

The trend of Microsoft besting Unix appears to be accelerating, noted IDC. Windows server sales climbed 4.7 percent in the fourth quarter of 2005, year-over-year, while Unix server revenues dropped 5 percent. Linux, meanwhile, posted double-digit year-over-year growth: fourth quarter 2005's numbers were 20.8 percent higher than 2004's.

But Windows' moving past Unix doesn't mean the former top dog is going to quietly slink away, IDC analysts said.

"We do not believe that any one platform will be in a position to force another platform out of the marketplace for many years to come," said Jean Bozman, vice president of worldwide server research, in a statement. "End users [will continue to] utilize a mix of operating systems in their infrastructures."