Microsoft CEO Steve Ballmer got lots of attention yesterday when he announced that the company has sold twice as many copies of Windows 7 as it did any previous version of the operating system in its first month. It's "a fantastic start," he told shareholders at the company's annual meeting.

People who have been around the industry a while tend to roll their eyes at these kinds of statements, because such comparisons don't take into account the organic growth in the computer market in the years between Windows releases. It's not hard to sell millions more copies of Windows when millions more PCs are being sold.

In fact, Microsoft floated the same claim about Windows Vista when it was released -- saying it "made a splash" with more than twice the sales of Windows XP in its first month. Ultimately, of course, Windows Vista didn't exactly set the market on fire.

But a closer look at the current situation suggests that Ballmer's positive outlook about Windows 7 might actually be justified.

For starters, the difficult economy and Windows 7's shorter development cycle mean that the new operating system doesn't benefit from as much underlying PC market growth as its predecessor did.

Windows Vista was released in January 2007, coming off a year when worldwide PC shipments were about 230 million -- making the market 70 percent bigger than when Windows XP was released. (There were 133.5 million PC shipments in 2001 according to IDC). In contrast, between Windows Vista's release and Windows 7's debut, the PC market has grown by only 30 percent, with current projections for 2009 somewhere in the realm of 300 million shipments.

That smaller market growth makes it notable that Windows 7 is still selling at twice the rate of Windows Vista.

That's not the only hurdle Windows 7 has needed to overcome in the comparison to Windows Vista. As noted previously by the Wall Street Journal, consumer PC sales are typically stronger in January than in October, which gave Windows Vista an additional edge.

For the record, Microsoft said in early 2007 that Windows Vista sold 20 million copies in its first month, which would mean the company has sold about 40 million copies of Windows 7 since the Oct. 22 retail release. Michael Dell, for one, is predicting that Windows 7 will help bring the PC market back to double-digit growth.

Windows 7 has had some factors working in its favor. Most notably, the new OS has been helped by pent-up demand from people who skipped Windows Vista because of its negative reputation. Overwhelmingly favorable Windows 7 reviews also helped.

Of course, it's important to remember that one month does not make a full product cycle. There's a long way to go before we get a true assessment of Windows 7's long-term acceptance in the market.

But in the meantime, this appears to be one case where Ballmer's cheery tone may be relatively well-grounded in reality.