You mean the boot process stops there? I'm surprised, because my motherboard is from '99, too, and I get the same message but booting just goes merrily on anyway.
The "acpi-force" the message refers to is an option you can put in the boot command to overcome this objection. The boot commands are located in a file in the /boot directory if your system has a /boot directory. In mine it's the file /boot/grub/menu.lst. However if you mess this up in the right (or wrong) way, you won't be able to boot at all from your hard drive, so if you're not a little bit experienced I think it might be better to leave it alone. If you are experienced, you probably already know how to change it.
You can also issue the boot command manually by stopping the automatic boot process (press the spacebar or almost any other key) before the automatic part begins. Then you will most likely see "boot" with a cursor after it and you can copy manually what you see as the first choice on the list of boot options, plus the "acpi=force" part. Acpi just shows battery status and temperature information, so if that's not important to you I'd just forget about it. But if the process is stopping at that point I guess you don't have much choice but to tell it to force acpi on.