Marcel Gagné

Hello everyone. I'm new to the forum so here goes. I recently installed Xubuntu via download cd to my older "NEC ready" (1998) 300mh. computer. It used to have Windows 98 on it. Well, when I boot up I receive the message, " ACPI=force is required to enable ACPI. BIOS age (1998) fails "2000" cutoff." The Xubuntu started and booted anyway, but it will not shut the computer down when I choose to shut down with the "shut down menu" in Xubuntu.
Not really sure what to do from here. Any help or suggestions would be helpful. Thank you in advance. Michael