Early sales of Microsoft's heavily hyped Windows Vista indicate that it will be the company's best selling operating system to date.

The software maker on Monday said that worldwide sales of licenses for the consumer versions of Windows Vista topped 20 million during its first full month of sales in February. By comparison, Windows XP, the predecessor to Windows Vista, sold only 17 million copies in its first two months of availability, according to Microsoft.

"We expected Windows Vista to become our fastest selling operating system ever, and we're on track for that," said Kevin Kutz, director of Windows client products at Microsoft, in an interview.

Consumer versions of Windows Vista officially went on sale Jan. 30. The numbers released Monday by Microsoft do not include sales of the enterprise edition of the product, which launched late last year.

Kutz declined to say how many of the more than 20 million Windows Vista consumer licenses shipped to date were sold directly to users who purchased a boxed copy of the software compared with licenses sold through PC manufacturers such as Dell and Hewlett-Packard.

Kutz also declined to say what percentage of the sales were attributable to purchases of premium versions of Windows Vista -- which feature Microsoft's slick new Aero interface and new navigation tools -- compared with the basic edition.

The sales figures include copies of Windows Vista that were purchased through an upgrade program that allowed consumers who had previously bought Windows XP-based computers to later upgrade their machines to Windows Vista for a nominal price.

Windows Vista's blockbuster early sales surprised some industry analysts. "Twenty million is a really big number," said Roger Kay of Endpoint Technologies. "It's especially interesting since we didn't exactly see people lining up around the block" to buy the software when it was launched.