While poking around in Resource Monitor, I saw that two of my CPUs, CPU 1 and 3, are marked as "parked", like so:
What exactly does this mean? I'm running Windows 7 x64 on a Core i3-530.
Windows automatically disables or parks cores to save energy in laptops. Even with the Power Plan set to full performance, CPU cores are parked by Windows.
You can disable it for better performance and poorer battery life. Instructions here and here. (Be careful, editing the registry is dangerous)