look in device manager, change the View to "By Connection", look at the
top item; does it say "ACPI Multiprocessor PC"?  If not you're not set up
right, you'll need to do an XP Repair Install.  BEFORE YOU DO THAT, check
in the bios for a setting named "APIC something-or-other"; APIC, not ACPI.  
APIC should be enabled, or XP won't use the ACPI hal, which won't allow
multiprocessor.  CHANGING the APIC setting will require a repair install,
don't just change it and expect things to start working. [the bios APIC
setting is slowly disappearing, so If you can't find it then it defaults
to enabled already].

If it does already say "ACPI Multiprocesor PC", there should be two cpus
listed underneath.  If they are there then all is well, check Task
Manager's Performance tab, the view menu should have a "one graph per cpu"
setting to see both cpu's loading.