Windows XP was one of Microsoft's greatest successes and greatest failures: A success because it was so rock-solid, a failure because it was so good that many people didn't want to upgrade to Windows Vista. Based on initial looks at Windows 8, Windows 7 may well be the new XP.
Windows XP was first released in 2001, and because of its speed, stability, and simplicity, has been a winner ever since. Even though it's going on 11 years old, it's still the most popular version of Windows according to Net Applications, with a 44.85% market share as of May 2012, compared to 40.51% for Windows 7 and 6.88% for Windows Vista.
Not everyone agrees that XP is the most popular version of Windows. As of May 2012, StatCounter has Windows 7 with 48.93% market share, and XP with 31.06% market share. It reports Windows Vista as having 8.38% market share.
Either way, though, it's remarkable that an operating system as old as XP has so substantial a market share. Also remarkable is how much more popular Windows 7 is than Windows Vista. That shows that when it comes to operating systems, people really do vote with their dollars. They've stayed away from problem-plagued Vista, and flocked to Windows 7.
My guess is that when Windows 8 comes out, people will also vote: They'll stay away from the new version of the operating system, and stick with Windows 7. Using Windows 8 on a traditional PC is simply too confusing because the operating system has been built for tablets more than for PCs.
Will Windows 7 manage to maintain market leadership 10 or more years after it was released? There's no way to know that. But I don't expect Windows 8 to overtake it, in the same way that Vista never overtook Windows XP.