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A required .DLL file, Glide2x.dll, was not found

From Quake Wiki

This error message may appear when you try to start GLQuake.

When you installed GLQuake, assuming you installed it properly, you had the option of also installing a 3dfx graphics card driver - specifically, the file opengl32.dll, which despite its generic-sounding name is actually a library designed to enable glquake.exe to work with 3dfx's Voodoo-based graphics cards.

This presence of this driver makes GLQuake dependent on another file, glide2x.dll, which is the Glide API library found in standard 3dfx Voodoo driver installations. That is, when you installed the drivers to make your Voodoo card work with your PC in general, glide2x.dll was probably installed as part of that package.

If you do not have a Voodoo-based graphics accelerator (you will know if you do), then simply remove the opengl32.dll in your Quake directory. This will make GLQuake start normally without looking for the Glide API library. You do not need the glide2x.dll!