GeForce 6600 detected as GDI Generic

I have a common OpenGL problem with a slightly different behaviour.

On startup it’s all as it should be:

Renderer Information
Manufacturer : NVIDIA Corporation
Video Adapter : GeForce 6600/AGP/SSE2
Version : 2.0.0
Video Acceleration : Yes, Hardware ICD
Multi-Texturing : 4 Unit(s)

Later on for unknown reason it reverts to:

Renderer Information
Manufacturer : Microsoft Corporation
Video Adapter : GDI Generic
Version : 1.1.0
Video Acceleration : No
Multi-Texturing : No

After reboot it’s back to normal then later again the GDI Generic takes over.

Running XP SP2 with the latest Nvidia 78.01 drivers.

Can I somehow reload the nVidia ICD without reboot, and/or what should I do to stop the Microsoft GDI Generic interfering with nVidia?

Do you have Matlab installed?