This is a copy from a old post on this forum.
Press winkey + R (opens the run command).inside type "msconfig" without commas.Go to the boot menu,click advanced options and tick the numer of processors box.Select 4 ok and then ok.reboot.
However,are you sure that only one core is running?By default the windows 8/10 task manager shows an overall load of the cpu and not the load per core.If you want to see what each core is doing,open task manager (ctrl+shift+esc),go to the performance tab and right click on the cpu graph(the big one) and select "logical processors".If you have 4 charts your cpu is running fine.
Worth a try.. 
