Quote "APIC - Win2k and XP can make use of the extra hardware IRQ's that have been commonly available in single processor motherboards over the last 18 months or so."
...and laptops got I/O APICs much later than desktops. You're assuming that the laptop in question is less than 18 months old. I suppose it might be - but I made the assumption that it was older, due to the Operating Systems that were being run. Toshiba Satellites have been around for years - I still use my 486-based one - and we do not know the age of this one.
FYI, ACPI is SOFTWARE like the BIOS is software (stored in hardware CMOS)
Windows 2000 has an ACPI driver to use it. Also, it doesn't matter if the laptop has 8 local APICs with 24 hardware interrupts each, without an I/O APIC, Windows 2000 reverts to 8259 PIC mode, ie 16 interrupts. Windows XP, on the other hand...
While those tweaking sites are good, I would advise checking out Microsoft and Intel's documentation on this subject.
The reason I think this discussion is O/T, although interesting, is because we ruled out interrupt sharing as being the cause as W2k reported no conflicts - instead it looks like a shortage of RAM. Even done through software in 8259 mode the speed of IRQ sharing is acceptable (if slow - according to Microsoft).