using System.Drawing;
protected void Page_Load(object sender, EventArgs e)
{
Image img = new Bitmap(100, 100);
Graphics g = Graphics.FromImage(img);
g.CopyFromScreen(new Point(100, 100), new Point(0, 0), new Size(100, 100));
IntPtr hdc = g.GetHdc();
g.ReleaseHdc(hdc);
img.Save(@"D:copy.jpg");
}