bloodline, is the APIC enabled in the BIOS?
Also - an APIC doesn't necessarily mean better performance or even better anything. I wouldn't get too cut-up about it if you can't get the system to work properly with it. I tested my Asus A7N8X-E Deluxe mobo with the APIC enabled, and none of my tests showed a performance increase, and there was an annoying onboard sound issue which could only be resolved by disabling the APIC.
I think the idea of the APIC is a step in the right direction, but it hasn't yet become a universal significant benefit. Kind of like SATA :-)
As for reinstalls - you can just change the HAL and reboot. In Device Manager > Computer > Update driver:
Advanced Configuration and Power Interface (ACPI) computer (this is just plain normal ACPI)
ACPI Uniprocessor PC (this one is ACPI + APIC)
It is possible that you'll have to reinstall other drivers afterwards, but I haven't had to yet.
Ok, I just noticed this:
I've upgrade my Motherboard, but kept my old WinXP installation
Cardinal mistake. Do not pass Go, do not collect $200 :-)
Have fun reinstalling (I'd advise a clean reinstall).
