Video Card doesn't support selected Opengl?

In a game I’m playing, when I change the video mode to Opengl it says “Your selected Opengl mode is not supported by your video card.” What can I do to fix this problem?

I think that Half-Life sometimes do this…

Anyway what you should do is to install
opengl drivers for your card if you don’t
alredy have them. You can use glsetup
for this.

If you have a proper driver. Use another
color depth or resolution.