If it is fullscreen, it is normal that the Taskbar cannot be activated.
Otherwise, you can reduce the size of 1 pixel to allow Taskbar activation (but it is not real fullscreen then...).
For example if the Taskbar is at bottom (its location should be checked first) :
MONITORINFO mi = { sizeof(mi) };
GetMonitorInfo(MonitorFromWindow(hWnd, MONITOR_DEFAULTTOPRIMARY), &mi);
SetWindowLong(hWnd, GWL_STYLE, GetWindowLong(hWnd, GWL_STYLE) & ~WS_OVERLAPPEDWINDOW);
SetWindowPos(hWnd, HWND_TOP, mi.rcMonitor.left, mi.rcMonitor.top, mi.rcMonitor.right - mi.rcMonitor.left,
mi.rcMonitor.bottom - mi.rcMonitor.top - 1,
SWP_NOOWNERZORDER | SWP_FRAMECHANGED);