ACPI Firmware access from Windows

Hi All,

How we can access the ACPI table in WindowsXP?
Is there any library or system call supported by WindowsXP
to read and modify the ACPI table?

I have to access/modify the ACPI table to enable the Windows
to provide the PCI Express expanded configuration space access.
One document "PCI-Express_Windows" from Microsoft site
describe this method. According to this document by modifying
the ACPI firmware we can make it possible to access the
PCI Express extended configuration space in Windows.

Thanks & Regards,
Sachin