public Form1()
    {
        InitializeComponent();

        pictureBox1.Image = Bitmap.FromFile("lena.png");
    }

    Point CurMouse;

    const int SrcWin = 20; //source window size
    const int DstWin = 80; //destination window size
    const int DistMouse = 30; //distance to mouse

    private void pictureBox1_Paint(object sender, PaintEventArgs e)
    {
        Graphics g = e.Graphics;
        g.InterpolationMode = InterpolationMode.NearestNeighbor;
        g.CompositingQuality = CompositingQuality.HighSpeed;
        g.SmoothingMode = SmoothingMode.None;

        g.PixelOffsetMode = PixelOffsetMode.Half;

        //zoom window (zooming image of picturebox)
        g.DrawImage(pictureBox1.Image, new Rectangle(CurMouse.X + DistMouse, CurMouse.Y + DistMouse, DstWin, DstWin),
                CurMouse.X - (SrcWin / 2), CurMouse.Y - (SrcWin / 2), SrcWin, SrcWin, GraphicsUnit.Pixel);

        g.PixelOffsetMode = PixelOffsetMode.Default;

        //draw text (should have inverted/good visible colors, depending on current background of zoom window)
        TextRenderer.DrawText(g, CurMouse.X + "," + CurMouse.Y, new Font("Arial", 12),
            new Point(CurMouse.X + DistMouse, CurMouse.Y + DistMouse), Color.Red);
    }

    private void pictureBox1_MouseMove(object sender, MouseEventArgs e)
    {
        CurMouse = e.Location;
        pictureBox1.Invalidate();
    }