After I installed Houdini Apprentice HD 9.1, I got the following error message:

Warning: OpenGL shader extensions not available. This is a limitation of your video card or driver, and will prevent Houdini from using GLSL shaders. Set the HOUDINI_OGL_ENABLE_SHADERS to zero to avoid attempts to use this extension.

I am using a Dell laptop with Windows XP Professional and a NVIDIA GeForce GO 6800 graphics card.

What does this error mean? How can I set HOUDINI_OGL_ENABLE_SHADERS to zero?