Boot into rescue mode with your Mandrake CD.
Edit your grub configuration file. I can't tell you what or where, exactly, since I'm using lilo, but what Manny would have done to enable APCI is passed a boot parameter to the kernel like acpi=yes, or REMOVED one like acpi=no.
If you can't find any reference to APCI, add either noapci or apci=no to your kernel boot parameters.
Sorry, I can't be more specific, I'm not down with grub.
--Rounan