The clock is ticking on Microsoft (NASDAQ: MSFT).

The world's largest software company has set a date for its long overdue update to its flagship PC operating system. Windows 8.1 will become available on October 18, offering new personalization features, the refreshed Internet Explorer 11 browser, and several other features that may help consumers get over the initial shock of last October's poorly received Windows 8 rollout.

Naturally, Microsoft won't call Windows 8 a flop. On paper, Mr. Softy has activated a ton of licenses. There has been no shortage of criticism for the learning curve involved in getting to know the platform that was built from the ground up with touch-based devices in mind, but the boo birds aren't as loud as we witnessed when Apple (NASDAQ: AAPL) picked Vista apart.

However, perhaps the reason why Apple isn't rolling out "I'm a Mac" ads to take down Windows 8 is that the pie itself is shrinking. 

Industry tracker Gartner reported its quarterly metrics for the PC market last month, showing that worldwide shipments of desktops and laptops fell a brutal 10.9% to 76 million units. This was the unprecedented fifth straight quarter of declines for the industry, and Microsoft can't hide from the cruel math. PC sales have grown less popular since Windows 8 hit the market.

Apple also isn't going to be taking shots at Windows because it's no longer Public Enemy No. 1 for the Cupertino tech giant. Consumers are flocking to tablet and smartphones where Google's (NASDAQ: GOOG) Android is the operating system of choice. 

Microsoft isn't a total loser here. It does collect patent royalties from Android. However, Microsoft has been a failure in the mobile computing markets, where its market share in smartphones and tablets clocks in at a mere 4%. There's little reason to expect that to change anytime soon, and its best shot at regaining relevance is to make sure that the PC market bounces back.

It won't be easy. The market has made up its mind, even though consumers are still buying more PCs than tablets. If Windows 8.1 doesn't win back mainstream users that are perfectly fine with surfing the Web, streaming media, and running apps on their "good enough" mobile computing gadgetry, will anyone still care about Windows 8.2 or Windows 9?

Microsoft isn't going away. It's too rich and entrenched for that to happen. However, Microsoft's gradual fade to irrelevancy is real. If October's update doesn't do the trick, investing in Mr. Softy won't be a treat.   

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Longtime Fool contributor Rick Munarriz has no position in any stocks mentioned. The Motley Fool recommends and owns shares of Apple and Google. It also owns shares of Microsoft. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.