It sounds like you still have an issue with the IO APIC, VT-x/AMD-v
If XP was installed as a single processor to begin with and you change it to SMP by turning on IO APIC and then VT-x/AMD-v flag, Windows will do exactly what you are seeing.
First can you boot in safe mode?
Make sure that the IO APIC is unchecked then boot in safe mode.
Tell XP to search for system changes. Make sure the the ACPI is not multiprocessor. (what it looks like with multiprocessor below)
If you can not get it to boot as single processor then you would need to follow these suggestions.viewtopic.php?f=2&p=84602viewtopic.php?f=1&t=24823&hilit=halu&start=15#p111502
I have had about a 50/50 success with the installation of multiprocessor in a single processor install.