When comparing operating system releases from Microsoft to current Linux distributions, you have to keep something in mind. Microsoft only managed to “keep” Windows XP for so long b/c the developers did a radical rewrite of the OS core for SP2. I have always considered Windows XP SP2 to be a completely different operating system from WinXP GM/SP1, and if you consider the history of the release, it becomes more apparent.
According to Wikipedia, Microsoft originally planned on releasing Windows Vista in 2004, or roughly 3 years after XP. That version of Vista was only going to be a minor upgrade and wasn’t going to include all the bells and whistles the real version of Vista has. Microsoft was then going to do a major release of Windows in 2006/7, which would have looked like something of a Windows Vista/Windows 7 combined release in regards to all of the changes made to the operating system and new features.
What happened is Windows XP GM and SP1 had so many security problems that Microsoft diverted a lot of the resources dedicated to making the original version of Windows Vista to “fixing” Windows XP. In the process, a lot of the core changes that were going to go into Windows Vista were backported to XP. Eventually SP2 for Windows XP came to be the interim operating system release for 2004, Windows Vista was promoted to being a major release (which was released in early 2007), and Win7 came to be a somewhat minor update of Windows Vista (essentially same core, but included some performance fixes and spiffy new features). So if you look at this time table of releases for Windows:
Windows 2000 - 1999
Windows XP GM - 2001
Windows XP SP2 - 2004
Windows Vista - 2007
Windows 7 - 2009
You can see that, in reality, Microsoft has been releasing new operating systems just as frequently as many of the enterprise Linux vendors do (roughly 2.5 - 3 years). Because Windows XP SP2 was a free upgrade to users, and had the same name as its predecessor, many standard users don’t consider it a new operating system. This leads users to think that Windows XP (specifically the SP2+ version that most people run today) has been around a lot longer than it really has.
Please note I left out the server variants of the systems, but those don’t typically effect the standard desktop user.
In summary, Microsoft really hasn’t supported Windows XP for 8 years because there is no single “Windows XP”. There were two releases under the same name, with the latter being 5 years old. Also, the original version of Windows XP is no longer supported (users are urged to upgrade), and the current version is only supported for security updates. So really, there isn’t as much of a difference between Windows operating system releases and enterprise Linux operating system releases as it appears.