Suddenly OGL Applications look strange under Windows 95

I´m going crazy!
OpenGL worked fine two weeks ago.
Now, if I run any OGL Application, e.g. Quake 2, all textures are rainbow - coloured and characters from the HUD or console are unreadable. I installed DirectX 7 and the latest “Detonator” drivers for my Riva TNT Board (Diamond Viper V550),supposing to solve the Problem but it still doesn´t work.
Have you got some hints to get along without reinstalling Windows 95? All suggestions are welcome (English, German)