1. Make sure you have second CPU added via VM Virtual Box Manager and "Enable IO APIC" checkbox in System settings on Motherboard tab is checked.
2. You should see 2 CPUs in Device Manager , but when you run Task Manager, you'll see only 1 CPU.
3. MS recommendation for Windows XP is to reinstall Windows (no surprise) as you can see it in this KB article: http://support.microsoft.com/kb/309283
4. Do not reinstall Windows, just run run the following from command prompt
rundll32 syssetup,SetupInfObjectInstallAction ACPIAPIC_MP_HAL 128 %windir%\inf\hal.inf
5. Reboot when prompted , log on back to Windows XP wait until you see "Windows has finished installing new devices. The software that supports your device requires that you restart your computer. You must restart your computer before the new setting will take effect" and press OK to reboot it again.