Why parallel processing doesn't work under Windows 8?

I have some codes written under Parallel Processing Toolbox and I have been able to matlabpool all the cores of my PC to run the parfor loops. This works under both Windows 7 and Mac, but not under Windows 8. In Win8, matlabpool still works to activate all the cores I have. However when I run the same codes, the CPU usage is only about 25% (I have an Intel i5) and the processing speed is significantly slower. Does anyone have the same issue like this? I am running Matlab 2013a.

Thanks!

[Edric Ellis]

On Win8, after opening the matlabpool, what does "matlabpool('size')" report as the number of workers you're running?