In most cases you will want to take a screen shot of the active window or the
Windows Desktop.

What if you need to do a screen capture of all the running applications -
most of them will be inactive and not visible to the user?

WindowSnap - Inactive Window Screen Capture

Windows XP also introduces
the new printing API, PrintWindow. This API enables the caller to snapshot a
visual copy of a window into a device context.

Drop a TImage (named "Image1") on a form and use the following code:

The actual WindowSnap function is defined as:

 uses Windows; //if not already used
function WindowSnap(windowHandle: HWND; bmp: TBitmap): boolean; var   r: TRect;   user32DLLHandle: THandle;   printWindowAPI: function(sourceHandle: HWND; destinationHandle: HDC; nFlags: UINT): BOOL; stdcall; begin   result := False;   user32DLLHandle := GetModuleHandle(user32) ;   if user32DLLHandle <> 0 then   begin     @printWindowAPI := GetProcAddress(user32DLLHandle, 'PrintWindow') ;     if @printWindowAPI <> nil then     begin       GetWindowRect(windowHandle, r) ;       bmp.Width := r.Right - r.Left;       bmp.Height := r.Bottom - r.Top;       bmp.Canvas.Lock;       try         result := printWindowAPI(windowHandle, bmp.Canvas.Handle, 0) ;       finally         bmp.Canvas.Unlock;       end;     end;   end; end; (*WindowSnap*) 

Note that the first parameter to the WindowSnap procedure is a HWND value (THandle) - the handle of the window you want to capture.

WinDowse is an extremely convenient and easy to use tool for obtaining necessary technical information about any window (handle, child windows, etc.)

Here's an idea: enumerate top-level windows (to grab their handles) and create your own task-switcher :)