I deleted the NVidia adapter from Device Mgr, moved the Microsoft versions of NV4.inf & pnf from Windows/Inf, NV4.dll from Windows/System32 and NV4.sys from Windows/System32/Drivers to a temp dir. On reboot to safe mode, it still showed the NVidia under video adapters, so I chose update drivers and pointed to the 3.69 Detonators. Lo, and behold - after reboot, DXDiag shows AGP and Direct 3D support. Several tries of just choosing 'update drivers' while the MS drivers were still in place had no effect, but all seems to be working now.