resetting number of cores

Hi,

I have an i7 4770 quad core processor, running with Windows 7 Professional. Looking in Task Manager, I could see 8 cores available, and also 8 ticked in msconfig/boot/advanced. I have a problem with cpu usage, trying to track down the cause. Earlier I reduced to 4 cores (although it was suggested I try 1), and now I want to revert to 8, but I cannot see how to do it.

I restarted after changing 8 to 4, and now only a max of 4 seems to be available.

Please could someone help?

Lyn