I am in the process of building a new PC and was having trouble with my
computer not shutting down completely when using Windows shutdown (I have XP
Pro) - the fans and drives would keep spinning and I would get a screen
saying "It is now safe to turn off your computer."

I was able to solve that problem using Microsoft Article ID #810903 (sorry -
don't know how to post link), but in the process discovered another issue: my
computer is being recognized as a "Standard PC" while I'm pretty sure it
should be "ACPI Uniprocessor PC."

The specifics on my machine are: AMD 64 3000+ processor, ABIT AN8 Ultra
Motherboard with their most recent BIOS, and Windows XP Pro. All my other
components are brand new and have up-to-date features/technologies.

Why doesn't XP properly recognize my machine? What functionality am I
losing or what problems might be created as a result of being a standard PC
vs ACPI compliant? The article suggests one can modify the HAL, but the
consequences of doing so incorrectly seem pretty high - how can I make sure
my computer is ACPI-compliant, as I think it might be, before taking this
step?