In 1990, A typical desktop computer had an Intel 80368SL and Trident TVGA9000 card.
In 1999, A typical desktop computer had an Intel Pentium III and integrated Intel video.
Since before 1995, some computers started having more powerful graphics card for 3D Gaming and Visualisation, but they were very rare. There were also monitors capable of displaying more than 1024x768 but they were expensive and not very common either.
http://www.computerhope.com/history/19902000.htm
Up until 2006, when Vista was released, most systems will had a Single-Core Processor and underpowered Integrated Graphics.
Vista (despite it's many flaws, and in some cases, because of it's many flaws), changed that. Dual-Core and 64-bit processors became ubiquitous and Accelerated GPU's from ATi and nVidia became necessities, rather than luxuries.
I shouldn't have said "all Windows Computer Hardware".
I should have said "the greater Majority of Windows Computer Hardware".
My Apologies, it was a gross generalisation.
Regardless, Developers only had to test against a handful of Systems. Nowadays with technology like DirectX and OpenGL, even high-end games benefit from the extra abstraction and shouldn't have to be too concerned about the specific Hardware.