The XP Subscription Model
The venerable Windows XP operating system is due to be completely retired in April 2014. First introduced back in 2001, it has stood the test of time, with many people still happily using it and holding a significant market share.
Microsoft obviously wants to move on and doesn't want to support code that's now well over a decade old. For many consumers XP is good enough though. Some are so familiar with it that the massive changes in Windows 8 forces them up a significant learning curve (and it's an OS that has been criticised for its lack of usability).
So why keep turning over new operating systems? Apple's OS X has been around about as long as XP and has required very few changes to the interface in that time. My suspicion is that, as one of Microsoft's big earners, they need to keep producing new versions so consumers and businesses will keep buying them.
So I'm proposing an alternative revenue model for Microsoft, so they can keep updating XP into the future: subscriptions. It seems to be the way they're going anyway, so why not offer existing XP users a £12 a year (£1 a month) subscription to keep XP? I'd guess you're looking at millions of existing users, so while that doesn't sound like much, it should be plenty to support a team of developers to keep releasing patches and updates.