Belying reports that the newest version of Windows is a poor seller, Microsoft ( MSFT) on Monday said that Vista is significantly outperforming its predecessor.

"In the first month of Windows Vista's general availability, sales exceeded 20 million licenses, more than doubling the initial pace of sales for its predecessor, Windows XP," the company said in a statement posted on its Web site.

That's 3 million more licenses than were sold for Windows XP in the first two months of its availability five years ago, Microsoft said.

Just five weeks ago, CEO Steve Ballmer said analysts' expectations of revenue generated by Vista sales were " overly aggressive " in some cases.

Microsoft cited reports from Dell ( DELL), which indicated that sales of the more expensive -- and higher-margin -- versions of Vista were selling surprisingly well.

The more than 20 million copies shipped represent Windows Vista licenses sold to PC manufacturers, copies of upgrades and the full-packaged product sold to retailers and upgrades from January 30 to February 28.

Even so, some PC makers appear in no hurry to retire from their product catalog computers running the old Windows XP, and there are anecdotal reports of consumer resistance to the hardware-hungry new operating system.

Shares of Microsoft were recently off 3 cents to $27.99.