Apple (AAPL) has been busy developing its next Mac operating system, dubbed Mountain Lion, and last week released an early version to developers.

The software will be made available for download from the Mac App Store in late summer, making Mountain Lion one of the company's fastest Mac OS rollouts to date. Normally, Apple follows a two-year cycle between consecutive upgrades.

What might have prompted the company to speed up the process? In a word, Microsoft (MSFT). (Microsoft owns and publishes Top Stocks, an MSN Money site.) We have a $550 price estimate for Apple, about 10% ahead of market price.

Windows 8 is coming

Microsoft, a longtime Apple foe, is planning to launch its Windows 8 operating system later this year. The software, built to work not only on traditional keyboard-centric devices but also touch-based ones, is Microsoft's attempt to challenge Apple in the fast-growing tablet segment.

The iPad has single-handedly cannibalized PC sales, leaving the industry with hardly any growth over the last few years. At the same time, the iPad and the iPhone have had a halo effect on Mac sales, helping Apple gain market share at the cost of Windows counterparts.

Although PC makers love to claim otherwise, the hard reality is that no competing tablet could pose any kind of threat to the iPad's popularity.

However, all that may change if Windows 8 becomes a hit and tablets running Microsoft's competing software become popular. The iPad may lose its aura and Mac sales may see a slowdown in their runaway growth.

Apple far ahead in mobile

Apple is leading the mobile revolution with the iPhone and the iPad, and is using that expertise to designing Mountain Lion. The company said it is borrowing some of the best features from its mobile operating system for use on the Mac. 

While Microsoft has been the 800-pound gorilla in PC software, its Windows Phone attempt have mostly disappointed. Things are starting to look up, if only a bit, only now due to a key partnership with Nokia (NOK). With the Windows Phone 8 (not to be confused with Windows 8) slated to release by mid-2012, Microsoft may finally have an answer to its mobile woes.

It will be tough for Microsoft to usurp market share from the firmly entrenched smartphone ecosystems of the iOS and the Android by Google (GOOG). However, if Microsoft manages to stake a claim on the still-nascent tablet market, it will be able to defend its PC business more effectively.