The software maker will automatically upgrade Windows 8 machines on October 17, the day before Windows 8.1 ships, but now Windows 7 users can pay $120 to order the Windows 8.1 upgrade DVD or a digital download.
"Windows 8.1 is not designed for installation on devices running Windows XP or Windows Vista," Microsoft cautions on the Windows 8.1 preorder page, but those customers can buy Windows 8 now and then get a free upgrade to 8.1.
Microsoft also will sell Windows 8.1 Pro for $199. The prices are for full versions of the OS, not just upgrades, which means people building their own PC hardware will be able to use them.
Windows 8 alienated many customers with an operating system that booted into a start page covered with high-contrast app rectangles rather than the familiar desktop with its start button. In 8.1, Microsoft reinstates the start button and gives users the option to boot their systems into the old-style desktop interface.
A lot rests on Windows 8.1's shoulders for Microsoft. Windows remains the dominant operating system on personal computers, but iOS and Android have captured not just the winning market share for mobile devices but also programmers' attention. With Windows 8 and now 8.1, the company is hoping to move to a new interface that's designed to be more secure, more power-efficient, and touch-friendly for tablets -- including Microsoft's own Surface tablets.
The Windows 8.1 launch will take place a few days before Microsoft tries a second time with its own tablet hardware, the $899 Surface Pro 2 and $449 Surface 2. The first-generation Surface products were largely a commercial flop, but the sequels bring new processors, new options for snap-on keyboards, and of course the new operating system.
Microsoft earlier this year released a Windows 8.1 preview version that gave people a chance to test the new operating system.
Upgrades could involve a lot of work. Windows 7 users and those who installed the Windows 8.1 preview version will have to reinstall their software with the final version of Windows 8.1.