However, the company is emphasizing two main SKUs (stock-keeping units) of the forthcoming OS -- Windows 7 Home Premium and Windows 7 Professional -- saying these are the ones most customers will buy, according to information posted on Microsoft's Web site.

With Windows 7, Microsoft hopes it will be easier for customers to decide which edition of the OS is right for them. By doing so, the company once again seems to be trying to learn from mistakes it made with the release of Windows Vista, premium versions of which had special hardware requirements that hindered customers' transition from XP and confused users as to which edition they should purchase.

When Microsoft released Vista, premium versions such as Windows Vista Ultimate and Windows Vista Home Premium did not run on many PCs that already were running Windows XP. Microsoft tried to give hardware partners and customers fair warning of this and through a controversial program had OEMs put stickers on new PCs to let customers know which version of Vista those machines could run.

Still, many customers did not feel Vista hardware requirements were communicated effectively and Microsoft is still embroiled in a class-action suit over that sticker program, called "Windows Vista Capable."

In addition to Home Premium and Professional, Windows 7 will come in the following editions that mirror Vista's SKUs: Windows 7 Starter; Windows 7 Home Basic; Windows 7 Enterprise; and Windows 7 Ultimate.

Microsoft is recommending either Windows 7 Home Premium or Professional for most of its customers, it said.

Windows 7 Home Premium is for the average user and Windows 7 Professional -- replacing Windows Vista Business -- is for small businesses and people who work at home but have to operate in an IT-managed or business environment with security and productivity concerns, the company said.

Windows 7 Starter is a limited-functionality version of the OS that will be available worldwide but only as a pre-install by OEMs (original equipment manufacturers). This expansion of its availability -- Vista Starter was only available in emerging markets -- led one blogger to surmise that Windows 7 Starter is the version Microsoft will push for the netbook market.

"This version will only be sold through PC makers to users," wrote Paul Thurrott on the popular SuperSite for Windows blog, which has a list of SKUs and the differences in functionality. "This suggests that netbook makers will choose this version, even in the US."