Self-Managed Threads or Thread Pool?

Question

Up until now all my apps have been single threads but now I would like to get into multi threading. I know most of the ideas between creating and managing your own threads and putting it up in a pool and letting the OS handle it. I know that in a thread pool 1 thread/processor can run. My question is lets say for example I use Nero to make a DVD compilation. On a 1 core it takes about 3 hours. Now if you have 2 cores then it takes about 1.5 hours and so on with the time cut in half for the number of cores. So what kind of threading is this. Is it a single thread that the OS has put the cores to work? Is it a threaded pool and the OS is distributing the workload between cores?

Thank you,
{KJ}