We've all been hearing it -- Microsoft Corp.'s (NASDAQ: MSFT) failure to win over consumers and business users with the Windows Vista operating system is causing sales of the older Windows XP operating system to stay afloat. Microsoft doesn't want this, of course. It's true that the software maker has shipped more than 140 million copies of Windows Vista, but since Vista is the default operating system on millions of PCs, it's pretty easy to do that.

Some corporate customers, though, have bypassed Windows Vista completely and will wait until the next round. This is Microsoft's Achilles' heel -- some companies won't fix something that isn't broke. General Motors Corp. (NYSE: GM) even says that "We're considering bypassing Vista and going straight to Windows 7," in reference to Microsoft's next operating system due sometime in the future. Yes, many large companies are indeed taking Windows Vista in -- but it's mostly due to not having much choice with changing out entire computing infrastructures for a global corporation. It takes a visionary IT leader to do that, and those are hard to come by in many cases.

And therein lies a big problem for Redmond. If customers aren't excited about its new operating system, why would they think Windows 7 will be any better? It's hard to fathom Microsoft pouring $5 billion into Vista and being shunned left and right. The software maker's operating system and Office productivity business subsidizes all its other products where it may make little or no money. But what if Windows is destined to become a slow-growth industry? If that's the case, where is Microsoft's growth engine going to come from in 2010? 2012? It's making gobs of money now. Will it last? When its main product underwhelms much of the market, the question has to be asked.

Although Microsoft Corp.'s (NASDAQ:MSFT) Windows Vista product has not been "on shelves" long enough to really get a feel for how the new computer operating system is going to fare over the long term, retail sales at Best Buy Co. Inc. (NYSE:BBY) are doing well, according to Citigroup retail analyst Bill Sims.

Sims said that "Best Buy is likely seeing better than expected sales of Microsoft Windows Vista-based PCs since their introduction on January 31." The key word here is, of course, "likely," which alludes to a probability that makes some accept a certain comfort level. But what about the normal sales cycle of PCs? Now that there are thousands (or millions) of PCs sitting on store shelves with Windows Vista already loaded, the normal seasonality in PC sales will, of course, drive sales of Windows Vista licenses.

There are also quite a few machines on store shelves that have the older Windows XP operating system on them -- so Best Buy is getting some extra cash from the PC makers to help sell through these machines, according to Sims. Once those are sold, then all new PCs (well, generally) will have the Windows Vista operating system pre-loaded -- and then the sales will start marching regardless.

When considering Microsoft CEO Steve Ballmer's comments yesterday on the revenue from Vista sales being initially over-forecast (liveblogged here), 2007 sales of Vista will probably just be a ramp for a sales increase in the future. The thing is -- investors want more immediate sales after five years in development.

[Disclosure: I own MSFT shares as of 2-16-07]