from: platforms/windows/qwindowscontext.cpp
QWindowsUser32DLL::init()
{
...
    updateLayeredWindowIndirect = (UpdateLayeredWindowIndirect)(library.resolve("UpdateLayeredWindowIndirect"));
    Q_ASSERT(setLayeredWindowAttributes && updateLayeredWindow
             && updateLayeredWindowIndirect);
As Windows XP does not have the api UpdateLayeredWindowIndirect, so the assert will fail.