Please help?

I it is necessary to draw polygon on screen, but me this it is necessary to do with screen coordinate.

For example:

// x=1024/2, y=768/2 (int type)

beside me this works only I happen to to use 3D system coordinates

// x=1.5, y=1.5 z=1.0 (float type)

Say please how do I realize this.

Sorry for my english.