This code snippet shows how you can move a borderless form.

private const int WM_NCLBUTTONDOWN = 0xA1; 
private const int HTCAPTION = 0x2; 

[ DllImport( "user32.dll" ) ] 
public static extern bool ReleaseCapture(); 

[ DllImport( "user32.dll" ) ] 
public static extern int SendMessage( IntPtr hWnd, int Msg, int wParam, int lParam ); 

private void Form1_MouseDown( object sender, MouseEventArgs e ) 
{ 
  if ( e.Button == MouseButtons.Left ) 
  { 
    ReleaseCapture(); 
    SendMessage( Handle, WM_NCLBUTTONDOWN, HTCAPTION, 0 ); 
  } 
}  

Contributed from George Shepherd's Windows Forms FAQ