HELLOWWIN wnd clas structure was set to the flags CS_HREDRAW &CS-VREDRAW .This directs windows to invalidate the wholev window when the size changes .The window procedure then receives a WM_paint message.
WM_PAINT processin almost begins with a call to begin paint .
hdc = Beginpaint (hwnd ,&ps );
and end with a call to end point ;
EndPoint ((hwnd ,&ps);
                First argument at handle to window ,pointer is  structure of type PAINTSTRUCTURE .
                If the window procedure does not process WM_PAINT messages they must be passed on to DefWindow  proc. DefWindow simply calls BeginPaint and end paints in succession so that the client area is validated.

Get Client rect (hwnd ,&rect)
                            The firs argument is the handle to the parameters window. Second argument is a pointer to a rectangle styructure of type RECT. This 
structure has four LONG fields named left, top, right, &bottom . left &top fields are always set to 0.Right &bottom feilds represnts the height &width of the client area in pixels.

DrawText (hdc,TEXT("Hellow windows 98!")-1,area+,DT_SINGLELINE | DT_CENTER |DT_VCENTER );