[lproven]

> For performance reasons it lived in the NT kernel, together with the Window manager (which also draws windows using GDI).

Only from NT 4 onwards.

NT 3.1, 3.5 and 3.51 ran GDI in user space.

NT 4 moved it into the kernel.

NT 5 (branded "Windows 2000") and NT 5.1 (branded "Windows XP") kept it there.

It is interesting to consider is as moving back out again; it never was, in my understanding, and even today in "Windows Server Core" it still has the window system built in.

But GDI was not so much moved back out of the kernel again as replaced in NT 6 ("Vista") with the new Aero Compositor.