Ok, the title says it all. I just got a Core 2 Duo 6300 and when I go into the task manager under the Performance tab, I only see one core. What's up with that? Other than that, the whole system is running fine.
When I go to the Device Manager, both cores are shown under Processors.
Have you reinstalled Windows? I'm not certain if it's true for XP, but under Windows 2000 you had to reinstall Windows if you added a second CPU. This ensured that it loaded the SMP kernel. Otherwise it runs, but only on one processor.