Microsoft's new browser, Internet Explorer 10 (IE10), will not run on Windows Vista, either now in its developer preview form or when the software ships, the company confirmed today.

The decision makes Microsoft the first browser developer to drop support for Vista, and follows the move last year when it announced Windows XP would not run IE9, the browser that went final four weeks ago.

In release notes published Tuesday, Microsoft said that users need to run the IE10 Platform Preview on Windows 7 RTW -- the designation for the original 2009 release of the OS -- or Windows 7 Service Pack 1 (SP1). The latter started reaching users on Feb. 22.

Windows 7 RTW must be updated before it's able to run IE10, said Microsoft.

When Vista users try to install the IE10 preview, they see a dialog box that reads, "Windows Internet Explorer Platform Preview does not support any operating system earlier than Windows 7," after which the installation process terminates.

Windows XP users see the same message when they attempt to install IE10.

Wednesday, Microsoft confirmed that the new browser is intended only for Windows 7.

"Windows Vista customers have a great browsing experience with IE9, but in building IE10 we are focused on continuing to drive the kind of innovation that only happens when you take advantage of the ongoing improvements in modern operating systems and modern hardware," a Microsoft spokeswoman said in an email reply to questions.

The spokeswoman also lumped Vista in with the 10-year-old Windows XP when asked why Microsoft was abandoning the former with IE10.

"Regarding Windows Vista, our decision with IE9 was not to build to the lowest common denominator," she said. "With IE9 we made the decision to help unlock the best Web experience possible, which means taking advantage of everything around the browser -- including Windows 7 and modern PC hardware."

Microsoft has been aggressively promoting the idea that a browser is only as good as the operating system it runs on, and that by extension, browsers that run on older OSes are sub-standard.

Last month, Dean Hachamovitch, who leads the IE team, went so far as to say that rivals -- presumably meaning Google and Mozilla -- "dilute their engineering investments" by creating browsers for the Mac, Linux and Windows XP.

Vista users were unhappy about the news that their machines won't run IE10.

"IE10 is not supported for Vista? Looks like IE9 is the new IE6," said one commenter, labeled only as "IE9 is the new IE6."