The world's No. 2 PC maker started selling Ubuntu Linux PCs to consumers in May in a program that was limited to the United States. Ubuntu is a free version of the software.

Linux software, the main rival to Microsoft's market-dominating Windows, has been one of the fastest-growing types of software on business computers over the past decade.

But it has yet to gain a foothold in the consumer market, where Windows sits on more than 90 percent of personal computers.

Dell says that so far the bulk of its U.S. Linux sales have to open-source enthusiasts. They tend to like the software because it is free, thousands of compatible programs are also free, and it is easy to customize.

But Dell says that a small number of Linux buyers are first-timers interested in trying out an alternative to Windows. If that group grows it could hurt Microsoft's profit growth.

Every PC that is sold with Linux installed on it instead of Windows means one less license fee payment from a PC maker.

Microsoft and PC makers don't disclose the size of those license fees. Retail versions of Windows Vista generally sell for $200 to $400.

On Monday No. 3 PC maker Lenovo Group said it would introduce a broad line of Linux laptops, the strongest endorsement to date of the open-source software by a major PC maker.

Dell said that the Linux machines it sells in Europe, which went on sale Tuesday, come with Ubuntu Linux.

Customers in China will be sold PCs factory-installed with Novell Inc's Suse Linux. The company did not say when those will go on sale.

Dell made the announcement in San Francisco at the annual LinuxWorld convention.