does your version of Bryce already know how to use the 4 cores? in that case maybe you can increase the priority of the process, not sure it that will work or make things difficult to operate, be careful with that, but sometimes if an app is competing for cpu cycles with another app it's fixable by going to the task manager (control-alt-del) and looking for the Process, right-clicking it and changing it's priority (try slightly above normal, don't go way too high or you might find it difficult to interact with WIndows until the rendering is done)