I would be all for getting rid of the windows tag, it is a meaningless way to tag a question. The windows tag could be applied to any windows products, it does not define the operating system that the question applies to. Used in combination with other tags, does not help much XP, 7, NT, 8, 98 , 95 , Server8 or whatever else there is.
The windows NT tag does specify a specific windows operating system, that has features and funtions that are unlike Windows XP or Windows 7 for example. But it is being applied for "nt" questions as it applies to the whole nt line of windows. It is not Windows NT, it is windows nt. It is not a synonym of win98.
There are huge differances between Windows nt (nt based), and Windows NT (the os), and between Windows Xp , and Windows 7 and Windows 2000. They should never be combined under a Lump.
This is important only for questions that are Just tagged, and the Q does not have the OS written into the question.
What is done with it, I do not know. I am just trying to cover what I see.