Thanks Rigoletto (you must excuse because thats the first shader I'm triing to program and it's very hard to learn with my bad english )

So here is my 2nd try

Code:

---

 material material_test
{
	effect
	"
		float4x4 matWorldViewProj;
		texture entSkin1;
		
		sampler ScreenProjSampler = sampler_state
		{
			texture = <entSkin1>;
			magfilter = LINEAR;
			minfilter = LINEAR;
			mipfilter = LINEAR;
		};
		
		float3 mainVS(float4 Pos : POSITION0) : TEXCOORD0
		{
		   float4 toScreen = mul(float4(Pos.xyz,1),matWorldViewProj);
			float3 tex0;
			tex0.x = 0.5f * (toScreen.z+toScreen.x);
			tex0.y = 0.5f * (toScreen.z-toScreen.y);
			tex0.z = 1.0f * toScreen.w;
			
			return tex0;
		}		
		
		float4 mainPS(float3 tex0 : TEXCOORD0) : COLOR0
		{
		   float2 screenCoords;
			screenCoords.x = tex0.x/tex0.z;
			screenCoords.y = tex0.y/tex0.z;
			float4 ScreenSpaceSample = tex2D(ScreenProjSampler,screenCoords.xy);
			
			return ScreenSpaceSample;
		}
		
		technique test
		{
			pass p0
			{
				VertexShader = compile vs_2_0 mainVS();
				PixelShader = compile ps_2_0 mainPS();
			}
			
		}
	";
} 

---

Now I get an error with the position in the vs and i can't understand why.