Although it is possible to manually modify the cores assigned to an application using the 'Task manager (core affinity), the freeware tool called Core Affinity Resident will automate this.
Core Affinity Resident is a portable tool. After executing the tool, left click on the tray icon, you will see all applications running in your system that are able to be reassigned to one or more cores.
Once your target application is selected, its core affinity will be shown. Now proceed to check or uncheck the desired cores and then push 'Save'.
Now your application will use only the cores you want to, and will remember your preferences in the future.
Download Core Affinity Resident (for Windows7, vista and XP)