The difficulty in striking this delicate balance became evident on Tuesday, when Microsoft officials spent the day at TechEd North America promoting Windows 8 in speeches, press conferences and demo sessions, while telling the 10,000 IT pros in attendance that their organizations, if they haven't done so already, should upgrade from Windows XP to Windows 7.

But at the morning keynote, when Microsoft officials made an aggressive case for enterprise adoption of Windows 8, they portrayed Windows 7 as an aging OS designed before key changes in computing happened in recent years.

"Windows 8 is a bold new bet, and it's a generational change in Windows," said Antoine Leblond, corporate vice president of Windows Web Services. "Windows 8 first and foremost is a better Windows than Windows 7."

Leblond said Windows 7 is the last in a line of OSes that began with Windows 95, designed primarily for desktop PCs that are always connected to a power source and act as the main repository for users' applications, data and content.

On the other hand, Windows 8 is designed for the world's shift to mobile devices that run on batteries and to applications and content that live dispersed in a variety of web sites and must be constantly available.