Vista and 7 were based on the same code-base and were basically forks of the same project. It is easy to split MS OS release cycles up into dates but the actual development/release process doesn't tend to follow that as elegantly as it is often depicted.
Originally, once we pass WFWG 3.11 and NT 3.51 (but really starting with those products) we get into the consumer/business split where MS had essentially two project developments running in parallel. Windows 95 and Windows NT 4.0 for example shared what was essentially the same GUI but with nothing under the hood being all that similar. The NT (Business) product benefited from being a true 32-bit OS with preemptive multitasking so that the kernel could control resources.
Development-wise, running basically "beside" that was Windows 95, the consumer "revolution" product from Microsoft which brought the GUI that most probably remember MS for. It was, like 3.11, a shell running on top of a 16-bit subsystem (MS-DOS).
Of course 95 came out before NT4. The NT products had a longer development/testing process and this would remain the case until the lines were merged later on.
The NT products benefited from long term support and service packs. NT4, released in 1996, saw its last Service Pack (6A) in 2001.
In contrast, its "consumer" sibling, Windows 95 saw a "B" release and then Windows 98, an overhaul of 95 with some more features slapped on (but the same shell on top of DOS setup), was pushed to market in 1997. It was incredibly buggy (I was on the beta testing team for it) and subsequently it was no surprise that a second release of the product, addressing some of the bugs was released (98SE).
It was around this time that the next release of Windows NT, Windows NT5, began development. This was another product I beta tested and have a few of the early builds, still labelled NT5, on CD somewhere. It brought updated DirectX support and to coincide with this MS changed the name of it to Windows 2000 and attempted to target consumers with the product to push them away from the DOS-based family of OS's. Unfortunately, due to the difference in how many things were handled differently in the NT OS, this meant that it didn't play well with a decent number of games. A big issue for the consumer crowd. This led Microsoft to develop yet another incremental consumer release product, Windows ME, which was essentially yet another revision of the Windows 9x family, but they did a better job of trying to hide the DOS subsystem, and they slapped the Windows 2000 GUI on it for good measure.
It was a disaster. I think it was possibly the worst Microsoft product most can remember. A lot of half-finished poorly tested code in it, Microsoft put very little effort into it and into supporting it and it showed. They wanted everybody onto the NT codebase which was simply better in every way.
After 2000 was released (or perhaps around the same time) its successor began development, NT 5.1, the product we know as Windows XP. Updated GUI, better directX support, improved compatibility efforts for legacy applications and legacy code, it was Microsoft's first earnest attempt at pushing the NT products into the consumer realm. It was an incredible success and became somewhat of an issue for Microsoft in terms of long-term support as most are aware.
As XP neared completion Microsoft had already started working on its successor, codename Longhorn, which was supposed to feature a fully active Desktop (the web on the desktop) and a myriad of new features like an entire new filesystem....etc. It was a very optimistic endeavour and as the project trudged on it dragged WAY behind schedule. Ultimately, needing to push a new product to market, Microsoft wrapped up the "completed" or near completed features of Longhorn and spun them into the product that we know as Vista.
Development on the Longhorn code continued however, now under the codename Blackcomb. Many of the features of Blackcomb were eventually ported back to Vista through the "Platform Update" release, which came after SP2. Many argued that it should have just upgraded to 7 as they were originally really the same product.
After MS had a mature product on the market (7), they then began developing 8. The Metro UI received many negative reviews but MS pushed it to market anyways. That ultimately resulted in the rush push for the 8.1 update, which still, since it eliminated the start button, resulted in yet another push to get a product that brought that back out there, which brings us to Windows 10. Windows 10 looks a heck of a lot like Windows 8/8.1. (I'm running it right now). But it has a start "panel" that is reminiscent of the start Menu that many so heavily criticized MS for removing in 8. It is a hybrid of the traditional start menu concept and the Windows 8 start screen with live tiles that you can move around, like with the start screen, but it is convenient and doesn't dominate your primary screen every time you press it like the obnoxious start screen did.
Since 8 came out in the middle of 2012 and we are now into 2015, we are certainly adhering to the "approximate" 3 year release cycle that is associated with MS products if we ignore 8.1. However the actual lifecycle for the products varies wildly with XP and 7 having massively longer lifecycle's than Vista for example, and of course we DO have that interim 8.1 "update" (2013), do we count this as a product release or no? If yes, then we are no longer at three years
I see 10 being the next "7" or next "XP", with 8/8.1 becoming the next Vista, which was the next ME.
This is why the enterprise and business world (and by extension, those of us who work in it) generally avoid the interim release products and wait until a product is dubbed mature before adopting it. Most skipped Vista and went straight to 7. Most avoided 8/8.1 and will probably adopt 10.
