Calling the situation "untenable" and describing Windows as "collapsing," a pair of Gartner analysts Thursday said Microsoft must make radical changes to the operating system or risk becoming a has-been.

In a presentation at a Gartner-sponsored conference in Las Vegas, analysts Michael Silver and Neil MacDonald said Microsoft has not responded to the market, is overburdened by nearly two decades of legacy code and decisions and faces serious competition on a whole host of fronts that will make Windows moot unless the Redmond, Wash. developer acts.

[ See why Microsoft may have dug itself into  too deep of a hole. And join 111,000-plus other users in InfoWorld's campaign to  save Windows XP. ]

"For Microsoft, its ecosystem and its customers, the situation is untenable," said Silver and MacDonald in their prepared presentation, titled "Windows Is Collapsing: How What Comes Next Will Improve."

Among Microsoft's problems, the pair said, is Windows' rapidly-expanding code base, which makes it virtually impossible to quickly craft a new version with meaningful changes. That was proved by Vista, they said, when Microsoft -- frustrated by lack of progress during the five-year development effort on the new OS -- hit the "reset" button and dropped back to the more stable code of Windows Server 2003 as the foundation of Vista.

"This is a large part of the reason [why] Windows Vista delivered primarily incremental improvements," they said. In turn, that became one of the reasons why businesses pushed back Vista deployment plans. "Most users do not understand the benefits of Windows Vista or do not see Vista as being better enough than Windows XP to make incurring the cost and pain of migration worthwhile."

Other analysts, including those at rival Forrester Research, have pointed out the slow move toward Vista. Last month, Forrester said that by the end of 2007 only 6.3 percent of the 50,000 enterprise computer users it surveyed were working with Vista. What gains Vista made during its first year, added Forrester, appeared to be at the expense of Windows 2000; Windows XP's share hardly budged .

The monolithic nature of Windows -- although Microsoft talks about Vista's modularity, Silver and MacDonald said it doesn't go nearly far enough -- not only makes it tough to deliver a worthwhile upgrade, but threatens Microsoft in the mid- and long-term.

Users want a smaller Windows that can run on low-priced -- and low-powered -- hardware, and increasingly, users work with "OS-agnostic applications," the two analysts said in their presentation. It takes too long for Microsoft to build the next version, the company's being beaten by others in the innovation arena and in the future -- perhaps as soon as the next three years -- it's going to have trouble competing with Web applications and small, specialized devices.

"Apple introduced its iPhone running OS X, but Microsoft requires a different product on handhelds because Windows Vista is too large, which makes application development, support, and the user experience all more difficult," said Silver and MacDonald.

"Windows as we know it must be replaced," they said in their presentation.

Their advice to Microsoft took several forms, but one road they urged the software giant to take was virtualization. "We envision a very modular and virtualized world," said the researchers, who spelled out a future where virtualization -- specifically a hypervisor -- is standard on client as well as server versions of Windows.

"An OS, in this case Windows, will ride atop the hypervisor, but it will be much thinner, smaller and modular than it is today. Even the Win32 API set should be a module that can be deployed to maintain support for traditional Windows applications on some devices, but other[s] may not have that module installed."