The following code snippet demonstrates how you can make your form cover the whole screen including the Windows Taskbar.

// Prevent form from being resized. 
FormBorderStyle = FormBorderStyle.FixedSingle;

// Get the screen bounds 
Rectangle formrect = Screen.GetBounds( this ); 

// Set the form's location and size 
Location = formrect.Location; 
Size = formrect.Size; 

Contributed from George Shepherd's Windows Forms FAQ