how do I fix full screen quad drift

If I have a 512x512 texture that I want to render a full screen quad into so that I can copy another 512x512 texture into it such that the pixels line up exactly, how do I fix the position/texture coords for this? currently I have (C#): TexturedQuadVertex[] quad = new TexturedQuadVertex[4]; quad[0].position = new Vector3(-1,1,0); quad[0].texcoord= new Vector2(0,0); quad[1].position = new Vector3(1,1,0); quad[1].texcoord= new Vector2(1,0); quad[2].position = new Vector3(1,-1,0); quad[2].texcoord= new Vector2(1,1); quad[3].position = new Vector3(-1,-1,0); quad[3].texcoord= new Vector2(0,1); where my positions are in projection space...but I get a drift to the bottom right when doing this in a feed back loop. I need the pixel to line up exactly.