Windows 2.1 (aka Windows/286 and Windows/386)
Windows 3.0, Windows 3.1, Windows 3.11, Windows 3.11 for Workgroups (WfW)
Windows 95 (Windows 4.0)
Windows 98 (Windows 4.1), Windows 98 Second Edition
Windows Millennium Edition (Windows 4.9)
Windows NT 3.1, 3.5, 3.51
Windows NT 4.0
Windows 2000 (Windows NT 5.0)
Windows XP (Windows NT 5.1)
Windows Server 2003, Windows XP 64-bit Edition 2003 (Windows NT 5.2)
Windows XP Professional x64 Edition (Windows NT 5.2)
Windows Fundamentals for Legacy PCs (Windows NT 5.1+)
Windows Vista (Windows NT 6.0)
so...next one being 7 sounds about right.
Now about that Pay-as-you-go plan...
People thought I was crazy when I suggested this was going to be the future of Windows when I mentioned it back in 2002.
At that time I had thought it would be that maybe you'd get a basic OS for free (like you can get Linux for free), with them passing out disks in much the same way AOL did (flooding the market with their 'free' product). They would probably only release a new version of the CD about once every 10 years, but continue to have newer versions of Windows as a series of 'patches' to be applied to the one that is available on disk, and them keeping the disk always behind the current version when they release a new CD.
And that to access Windows Update to get these 'patches' would be on a yearly or monthly subscription, which all patches and upgrades to bring you up to the current Windows version would be offered, once you paid. Without the fee being paid, it is possible that they would cause your OS to 'expire' and they would have some way of shutting it down till you called and gave credit card info over the phone. (much in the same way you pay for internet access and it gets shut off if you don't pay your bill)
If they offered it this way, they would have much less problems with piracy on their hands, and less of a problem with people running older more insecure versions, because they could force newer versions on people... but at the same time, there would be no way people of lower incomes would be able to afford to run it.
It is also possible that on an older machine that couldn't handle an upgrade, that the OS would shut down permanently, forcing you to buy a new computer if you wanted to continue using Windows.
As I can see now, they aren't going this far...yet. (at least not with their next version of Windows)