It's actually quite difficult to disable core on a computer if you don't know what you are doing. First we need to make sure that you are in fact only running 1 core.
We can do this through several methods.
I'll run through a few.
Open the device manager. (windows key/start button and type Device manager.
Open the icon under the control panel listing)
In the device list you'll see a title called processors.
Open it.
If you see 8 entries you are running all 8 cores (your i7 is a quad core with hyperthreading which Windows recognizes as 8 core)
Open the task manager (ctrl + shift + exc), go to the performance tab.
Here you'll see the CPU and memory use in real time.
Under view at the top, mouse over CPU History, select one graph per CPU.
The number of small graphs will determine the number of cores active and their activity levels. if you have 8 graphs and 1 or 2 with activity, your cores are enabled and just waiting for tasks to run.
Lastly, if you have less than 8 graphs in the task manager or CPU entries in the device manager, you may wish to check that the cores are enabled during bootup by the OS.
Open msconfig.exe (windows key/start button, type msconfig, open the icon), go to the Boot tab.
Click advanced options.
Make sure that either the check box labelled Number of Processors is either OFF (uses system default of all) or on and the drop down list shows a selection of up to 8 and 8 is selected)
Apply these settings (if you have changed any) and reboot.
Check again using the 2 methods I showed previously and make sure that you have the 8 cores available.
Just because you have 8 cores available doesnt mean you have to have them running. Under low loads your cores will basically disable themselves to reduce power consumption and heat generation. This is normal. as soon as there is processing to be done they will be active.
If however you still can't get them to work, you may have a BIOS or hardware issue. For this you'll either need to go into the BIOS during bootup and check stuff out in there (I can't give specific instructions because different motherboard manufacturers have different BIOS versions for different models and they have a very wide range of usability and control).
If you find that you are unable to activate your cores through BIOS you may need to contact technical support from your computer manufacturer.
(if you built it yourself, contact your motherboard manufacturer)
EDIT:
I forgot about core parking. Since I'm on a mobile device now i'll just link you to a video explaining how to fix this.
http://www.youtube.com/watch?v=20v3lZuUi1s