My computer (HP xw6600 Workstation) has two Intel Xeon processors which have 8 cores each, but system manager and performance manager only see four cores.
The Device Manager shows eight Intel(R) Xeon(R) CPU E5410 @ 2.33GHz.
They are all installed and drivers working perfectly but still only four cores are in use.
How can I get Windows to use all the cores?
Might be core parking you are seeing. Basically the os decides it doesn't really need all 8 at the moment and shuts some down to save power. The link I provided has some ways to turn this off if you want, but from what I understand in theory it only does this when it determines there's no performance benefit to be gained from the extra cores.
Reminds me of a modern dodge hemi v8 5.7 with a feature called MDS which goes into 4 cylinder mode while maintaining speed on the highway for better fuel economy. But as soon as you accelerate again its back on all 8.
Windows 7 Home Premium and Home Basic are limited to one physical processor. To use two physical processors, you will need to run at least Windows 7 Professional. As you didn't mention the edition of Windows you're running, that might be the problem.
