PC-98 is/was a Japan-common computer in the 1990s, similar to the IBM computers in America. For more info, click here.
All Touhou games from Highly Responsive to Prayers (TH01) to Mystic Square (TH05) were developed for the PC-98 platform. Among the fan community the term "the PC-98 games" refers specifically to these first five games. They are distinguished from "the Windows games" (from TH06 and on) for having been all made by Amusement Makers.