Similar problem: Have a server, DXDiag shows as ATI MACH64 but running generic windows driver works fine for most part, have resolution set 1024x768 has higher option but not supported by monitor, monitor does do much higher res just not that one. trying to load PowerDVD12 and get an error need to load proper driver for video.
ATI did manufacture those cards as ATI. AMD/ATI no longer seems to have these drivers available unless supported by one of their driver packs which I am currently in process of checking. Trying to search for some of the legacy cards on the AMD site they just arent there. Quite possible will have no support in XP