Microsoft's next version of Windows will be a lot more modular. ARS Technica writes Windows 7, which is expected in 2010, will likely be available in pieces and may work with subscriptions:
Unsurprisingly, Microsoft already has a patent on a "modular operating system" concept. A "core function" module, which includes the kernel, features a "license validation module" that authorizes the use of all additional modules, and uses DRM-like technology to prevent the use of unauthorized modules. Windows Vista uses part of its anti-piracy arsenal to validate and monitor changes to the OS for this reason.
So, Windows 7 will be modular, but to an unknown degree. I personally expect the modularization to focus on value-adds, as did Anytime Upgrade on Vista. It allows Microsoft to draw lines between what is and isn't "in" the OS for DoJ compliance issues. Whether it be Live Services, Windows Media Player, or even Internet Explorer, Microsoft could roll those into modules and then say, "Hey, look, that's not part of Windows, we're charging extra for that!" Foley says that she's heard from sources that Microsoft is working on a Photo + Mail + Video module that would exist apart from the OS, for instance. I've heard less specific groupings myself.
A modular approach could also allow the company to make functionality available on a time-limited basis, potentially allowing users to "rent" a feature if it's needed on a one-off basis. Note that Microsoft is already testing "pay as you go" consumer subscriptions in developing countries.