If I'm not mistaken, the 1.0.2 patch puts the sound processes on a second core.
Otherwise, with the task manager open, go the processes tab, then right click on the process you want to change. A drop-down menu will appear, and one of the options says "set affinity". Click on it, and you will be presented with a list of cores.
On my system, a quad core, I get an options to use all processors, or CPU 0 thru 7. Anybody else know why there are more CPUs than I have?