Why won't Windows use the other CPU cores?

Many apps don't use all cores. They aren't programmed for it. Then they will use one core and will overload it.

But you can tell Windows to run that process on 2,3, ... and in all cores.

For doing it, follow these steps:

1. Open Task Manager
2. Select tab Processes
3. Right click and select 'Set Affinity'
4. Select on which cores you want to run process.

your processor is quad core it will run 8 threads on multithreading however not all applications are programmed to use multiple threads some applications just require 3 threads so it means that application will use only one core or two. so even on setting the affnity of process for 4 cores it will still use less hardware

If you have Intel Core i7 2600 CPU first check if it is Dual Core or Quad Core.
Some high end Core i7 CPUs are 6 core processors , and the other cores you see in Task Manager performance tab are virtual cores.