Graphics are glitchy, but not in demo

[Indecom4000]

I played the demo a while ago and it looked and played well. Nice lighting, shadows and bumpmapping. No problems with it at all.

But, now i have the full version and it tells me that i dont support vertex shaders, and on low quality (not very low) I get strange zbuffer errors with millions of black lines running across all the unlit models in the game.

I have a geforce 4 ti 4600 and an amd xp 1800+ with 256 megs of ram.

Does anyone know of a fix or why the full version is glitchy and not the demo?