@Smiggy - What CPU do you have exactly (e.g. mine is an i5-2500K)? It could be that you're using Turbo Boost at it's highest frequency, which on some models means all but one core is disabled (to reduce heat). That sounds pretty likely. Check your BIOS and try disabling Turbo Boost.
Also, what does Windows Task Manager say about all of this?
Also, what does Windows Task Manager say about all of this?