OGL: GL_QUAD rendering wierd

I'm using a class to Display a square, (square class)

Code:

---

void square:: DisplaySq()
{
        glRotatef(theta, x[0]+(width/2), y[0]+(width/2), z[0]+(width/2));

        glBegin(GL_QUADS);               
                for (int i=0; i<4; i++)
                {
                        glColor3f(r, g, b);
                        glVertex3f(x[i], y[i], z[i]);
                }
        glEnd();

        glFlush();
}

---

I'm using the NeHe "backbone" the lesson1 cpp that sets up OGL for me. In this I added the RenderScene() function. In here all I do it call DisplaySq();

Code:

---

void RenderScene()
{
        s1.DisplaySq();
}

---

But heres the problem, when I view the program in windowed mode it doesn't show the square. When I go fullscreen (F1) I very lightly see it. Its like its being scanned (like a television does) almost transparent, just lines.
I'm guessing its clearing/refreshing the screen real fast and it taking the quad away and placing it back real fast it causeing this effect, so... how do I fix this?

Thank you in advanced!

NOMAD