If it's already stating you have a mutliprocessor pc then you simply need to go into your task manager and select "view/cpu history/one graph per cpu".
If device manager is not showing a multiprocessor pc the you may need to reinstall XP. You can also try going into device manager, selecting whatever is listed under "computer", right-click on it and select "update driver". Then you'll need to choose the correct HAL, most likely "ACPI Multiprocessor PC". Using this method sometimes renders the OS inoperable, in which case you'll need to reinstall the OS.