Well, please DO NOT try this on a multi proc. machine, it will NOT WORK ...
I have tryed this a while ago, and to really change the Ms HAL (which is different for acpi) you must reinstall.
But acpi is not that evil, just has to be configured properly.
A machine with a million shares work bad anyway. acpi or not.
acpi uses normally irq9, which actually is a crippled irq2 ..
and normally you don't want to use this irq to setup system devices...
the best is just to disable com's/LPT shit, and use 3x more irqz
just my 2c
have fun with acpi