In July, Kevin Turner, Microsoft's COO, told financial analysts: "By our math, we eclipsed the entire install base of Apple in the first five weeks of shipment."

Turner said there are 42 million PCs covered by volume licensing annuity agreements for Vista, and said strong renewals for Enterprise Agreements (EA) were driving the uptake.

He said customers are purchasing the rights to Vista because they are planning deployments.

But IDC's Gillen notes that purchasing rights to Vista and rolling it out are typically two different events.

Microsoft back tracked on its optimism Thursday in statements by Mike Nash, corporate vice president for Windows product management, which were posted on the company Web site.

"Most of our previous operating system releases were available for about two years after the new version shipped, so maybe we were a little ambitious to think that we would need to make Windows XP available for only a year after the release of Windows Vista," Nash said. The previous day, however, in an interview Nash did with Microsoft's own Channel 9 Web site, he said "nostalgic users" longing for XP were partly to blame for questions around Vista's uptake.