I thought I'd post this here in case it ever helps anyone else trying to do this.
A while back, I bought an AMD Opteron 185 dual-core to replace my beloved Fx-55. I do a fair amount of 3D rendering and those programs are often multi-processor friendly. I have a dual-boot system: XP and Vista Ultimate x64. Unsurprisingly, Vista saw both cores on the next boot after the processor install. XP did not.
XP just wouldn't see the other core (though ironically it would show two processors in the Device Manager, but it would report only one during boot and only ever use one core). It seems that when I installed XP, it picked a HAL that didn't offer MP in the Device Manager, so just changing that and rebooting was not an option. I found forums describing how sysprep could be used for this purpose, and after one such attempt and an un-bootable system, I abandoned that approach.
Microsoft even has a KB article telling you "you can't get there from here." They suggest reinstalling Windows XP. Sigh. Rather not, thanks...
And so it was for several weeks: XP using one core, Vista using both.
Then, finally, I found that the Microsoft tool devcon (Device Console) allows one to do all sorts of nasty things to the HAL, including changing it to support MP. There's also an MSDN page on how to do exactly what I was trying to do (in complete contradiction to the KB article that tells you it can't be done).
Anyway, here are the two lines that changed my HAL so I could use both cores:
devcon sethwid @ROOT\ACPI_HAL\0000 := +acpiapic_mp !acpiapic_up
devcon update c:\windows\inf\hal.inf acpiapic_mp.
Then reboot. Twice (one extra after XP does some contemplating).
That's it... now I have both processors available in XP and Vista.
The MSDN page telling how to do it is here:
The relevant MS KB are:
For the record, mine was "Advanced Configuration and Power Interface (ACPI) PC", ACPI PIC HAL (Halacpi.dll)
... which, as you can see from the second KB article, doesn't support MPs.
Anyway, hopefully having this here may save someone the trouble of tracking down this incredibly simple approach again.