For all of its astounding profitability and presence, Microsoft's most important product, Windows, has had a tough couple of years. And the path ahead, which Microsoft plans to lay out at a conference this week, is fraught with challenges.

When the company divulges details of Windows 7, the successor to much-maligned Vista, it will do so against a backdrop of growing competition from Apple, a battered Windows brand and the global economic crisis. Meanwhile, the company is positioning itself for the next big change in computing, which could undermine the importance of the traditional operating system as people use more programs through the Internet.

Today, Microsoft's operating-system software is thriving. It runs at least nine out of every 10 computers. Vista, for all the criticism, has sold more than 180 million copies since hitting the market Jan. 30, 2007. In the past quarter alone, Windows generated sales of $4.2 billion, 28 percent of Microsoft's revenue. As it has for years, Windows is fueling the company's expansion into newer businesses, including entertainment and Internet search.