I figured as much. It just didn't make sense otherwise.
Is that an AMD or an Intel CPU? And XP Pro, I assume, clean install?
If it's an AMD you need to install the Dual-core optimizer driver on this page; I needed it for all of my Opteron servers when they were running Windows:
AMD Athlon™ 64 X2 Dual Core Processor Utilities and Updates
If it's an Intel, try going to device manager>Computer and attempt to update the driver listed there. If I remember correctly you'll get some choices of kernels; just select the one that is ACPI SMP Processor, or something to that effect. Then see if it detects the cores correctly.