When Steve Ballmer launched the consumer version of Windows Vista in January, he boasted that the highly touted operating system was compatible out of the gate with hundreds of software applications from major independent software vendors. Consumers could rush out to their nearest Best Buy or Circuit City, buy a Windows Vista PC, and expect to run all of the latest productivity and entertainment programs without a hitch, Ballmer said.

What the Microsoft CEO failed to mention, not surprisingly, is that absent from his list of Windows Vista-friendly ISVs were the makers of some of the world's most widely used and important programs. Some products made by Adobe, IBM, Symantec, and a number of other high-profile developers were absent from the list of applications that Microsoft says are either fully certified for Windows Vista or will, at the very least, run relatively trouble free on the new OS.

Despite the omissions from its list, Microsoft officials insist that Windows Vista isn't suffering from a compatibility gap. They maintain that the current number of applications certified for the operating system is about what should be expected for a product that has been on the market for nearly three months. In addition, more applications are being added to the list all the time.

"You have to remember that our ecosystem includes hundreds of thousands of applications," said Dave Wascha, the company's director of Windows client partner marketing. "When we look at our internal metrics, we're way ahead of where we were with Windows XP at the same time," he said.

To emphasize the point, Wascha noted that Microsoft first started reaching out to ISVs whose applications would need to run on Windows Vista back when it was still just a gleam in the company's eye -- as far back as 2003, when Microsoft kicked off the first of what would be hundreds of strategic design reviews on the technology. "We highlighted what changes we were planning, and how we may have to work with partners to do some of that [compatibility] work," said Wascha.

Throughout the Windows Vista development process, Microsoft invited software engineers from hundreds of key application makers to Redmond for formal and informal meetings. There, visiting programmers would gather daily in a building about 100 yards from where Windows Vista was undergoing production. "There were engineers going back and forth between buildings, impromptu whiteboarding sessions were held, whatever was required," said Wascha.

Microsoft also did its own, automated testing of Windows Vista for compatibility with 1,200 to 1,400 major consumer and business applications, performing as many as 35,000 tests per week in the run-up to the product's January launch. "The end result is a couple of million automated tests over the development lifetime," said Wascha.

The stats are impressive, but also beg the question: With such an extensive compatibility program in place, why are some key software programs still not fully certified for Windows Vista?

It turns out that updating applications for Windows Vista is a significantly more complex a task than what faced ISVs when they needed to revise their products for the move from Windows NT and Windows ME to Windows XP in 2001. The most challenging problem, according to ISVs and Microsoft officials, is to create applications that will work well with Windows Vista's advanced security features, such as the User Account Control.