Three years ago mentioning those three companies in the same sentence would seem odd. Even today it may seem premature to assume that the worlds largest software company, a search company (although they're more media company now), and a company best known for design software to be involved in the next generation of the web, but that's where we are. And these three companies are fighting for not only your computer, but your mobile phone and your living room as well.
Microsoft is successful precisely because they have been able to leverage their operating system dominance into other markets. Now the brains in Redmond are in what to them must seem like bizzaro world. Software, where the margins are thick and major competitors thin, has suddenly been turned on its head. Software as a service is the new buzzword and companies with no background in software are releasing applications delivered over the web. All are aimed to compete with the core offerings from Microsoft. Microsoft is in a difficult position. Vista is publicly derided as a disaster, and they risk getting away from what pays the bills if they delve too far into the new software model. But they're doing an admirable job. WPF/E, or Windows Presentation Foundation Everywhere is positioned to play for dominance in the RIA market. They're making sure that developers who build applications on WPF can deploy those applications almost anywhere, on any device or OS. Similarly, the Flash Player has rebranded itself since the dark days of skip intro buttons and has become a full fledged development environment with players that will be put on everything from PlayStation Portables to regular desktop computers. Building an RIA and being able to deploy that RIA on a PSP is a compelling notion. Imagine being able to "link" your PSP to your XBox360 and your computer. It opens up the living room in exciting ways.
Through all of this I think Google has the most to gain, and is the farthest behind, from a rethinking of the operating system. They have been busily buying companies and building web applications that could change the way we all do business. If businesses are creating all of their documents with Google software and storing those documents on Google's servers, then it gives the search company the kind of leverage that Microsoft enjoyed in the 90s. It also means a lot of people possibly clicking on ads or paying a service fee. Google has so far relied on straight web applications - delivered in a browser and functional regardless of the operating system - but they lack the richness that makes desktop applications so user friendly. Their structure allows them to move more quickly than other companies, but up to this point they have eschewed the Rich Internet Application in favor of a more lightweight version which is high on functionality but lower in experience.
In the end, the user experience is going to win out. The starting point in this game is already that your applications will run anywhere. While reading that is probably disconcerting for those who know and love Microsoft, their WPF/E shows that they realize they need to be everywhere. Because of that, users are going to be drawn not just to applications that "just work" but to those that "just work" and also are pleasant to use. Flash and Windows Presentation Foundation are going to provide this kind of richness, and as of right now, Google simply isn't embracing that kind of richness. They may shift strategies as Rich Internet Applications become more mainstream, but Microsoft and Adobe have a head start. That head start is going to be a significant competitive advantage going forward.