begin
 instance :=GetModuleHandle(nil);
 
 WindowClass.style:=CS_HRedraw or CS_VRedraw;
 WindowClass.Lpfnwndproc:=@windowproc;
 WindowClass.Hinstance:=Instance;
 WindowClass.HbrBackground:= color_btnface;
 WindowClass.LpszClassName:='DX';
 WindowClass.Hcursor:=LoadCursor(0,IDC_ARROW);
 
 RegisterClass(WindowClass);
 
 Handle:=CreateWindowEx (0,'DX','',WS_POPUP, 5,5, 300, 200,0,0,instance, nil);
 
Тоесть предлагаешь изменить 
begin
 instance :=GetModuleHandle(nil);
 
 WindowClass.style:=CS_HRedraw or CS_VRedraw;
 WindowClass.Lpfnwndproc:=@windowproc;
 WindowClass.Hinstance:=Instance;
 WindowClass.HbrBackground:= color_btnface;
 WindowClass.LpszClassName:='DX';
 WindowClass.Hcursor:=LoadCursor(0,IDC_ARROW);
 RegisterClass(WindowClass);
 if RegisterClass(WindowClass)=0 then Halt(255);
  Handle:=CreateWindow(AppName, 'Flash disk detector', ws_OverlappedWindow,
    cw_UseDefault, cw_UseDefault, cw_UseDefault, cw_UseDefault,
    0, 0, HInstance, nil);
  ShowWindow(Handle, SW_MAXIMIZE);
  UpdateWindow(Handle);