Another test I've done is to enable boot logging with Win XP. This appends command lines to the file 'ntbtlog' in the windows directory, which mainly shows drivers being loaded as Win XP is launched. The log is normally date and time-stamped for every boot-logging session.
When all 3 sticks are installed, I cold boot, then press F8 and select 'enable boot logging' from the options menu. This is what appears on the screen as the system, attempts to load Win XP.
Loaded driver \WINDOWS\system32\ntoskrnl.exe
Loaded driver \WINDOWS\system32\hal.dll
Loaded driver \WINDOWS\system32\KDCOM.DLL
Loaded driver \WINDOWS\system32\BOOTVID.dll
Loaded driver ACPI.sys
Loaded driver \WINDOWS\System32\DRIVERS\WMILIB.SYS
Loaded driver pci.sys
Loaded driver isapnp.sys
Loaded driver viaide.sys
Loaded driver \WINDOWS\System32\DRIVERS\PCIIDEX.SYS
Loaded driver MountMgr.sys
Loaded driver ftdisk.sys
Loaded driver PartMgr.sys
Loaded driver VolSnap.sys
Loaded driver atapi.sys
Loaded driver disk.sys
Loaded driver \WINDOWS\System32\DRIVERS\CLASSPNP.SYS
Loaded driver fltmgr.sys
Loaded driver sr.sys
Loaded driver PxHelp20.sys
Loaded driver KSecDD.sys
Loaded driver Ntfs.sys
Loaded driver NDIS.sys
Loaded driver Mup.sys
Loaded driver BTHidMgr.sys
Then the process freezes and a solid white block appears in the top-left of the screen, with the top-right corner of the block blinking.
At this point, I turn off the PC using the main power switch, then remove the 3rd memory stick. After booting up again, with only 2 sticks installed, opening the text file 'ntbtlog' reveals that this boot event wasn't logged.
So, I reboot Win XP with only the 2 sticks installed, and select 'enable boot logging'. This time, the above list of loaded drivers is shown in the text file, plus a whole lot more. The potentially interesting observation is the next in the list after "Loaded driver BTHidMgr.sys" which is "Loaded driver \SystemRoot\System32\DRIVERS\amdk7.sys".
'amdk7.sys' is a driver file which seems to come with one of the Win XP Service Packs, and which "is necessary to enable Windows XP support for AMD PowerNow! technology on computers that use a mobile Athlon 4 or mobile Duron processor." http://support.microsoft.com/kb/306458
Various other posts refer to it in relation to AMD Athlon XP 3000+ Mobile, whereas mine is a 3000+ in a desktop. Also, if 'amdk7.sys' isn't available, then Win XP will load the generic driver 'processr.sys'. In fact, my 'ntbtlog' includes the entry "Did not load driver \SystemRoot\System32\DRIVERS\processr.sys" which would make sense.
However, my curiosity is that with the 3 sticks installed, Win XP fails to continue loading when reaching 'amdk7.sys' in the driver list.
Another test I did was to change the power management settings in the BIOS.
"IPCA Function: This item is to activate the ACPI (Advanced Configuration and Power
Management Interface) function. If your operating system is ACPI-aware, such
as Windows 98SE/2000/ME, select Yes. Available options: Yes, No."
This was normally set to Yes. When I set it to No and rebooted with 3 sticks installed, instead of a half-blinking white block (as described above), the screen went black and then the system rebooted itself...
The plot thickens!...
Maybe there's a clear explanation for this - however, I'm wary of doing something to the 'amdk7.sys' driver file (whether removing or disabling it somehow) in case it prevents Win XP from launching with just my 2 sticks installed. I'd guess there are precautions, but need further info on whether this is relevant, and what the implications might be.