Don't use cgminer. Use pooler's cpuminer aka minerd.
Here is original thread.
https://bitcointalk.org/index.php?topic=55038.0There are probably more up to date releases but without knowing your OS I don't want to go searching.
In miners you can use the -t call to specify the number of threads. So if you have 4 cores you could do "-t 3" and have only 3 threads. This does not leave an entire core open but it should have all four cores at ~75% usage. At least that is my experience.