The link sent by mcnumpty23 will answer your question. To explain in detail, you should open up the game, and when it loads up till the main menu, press the 'Windows' button on the keyboard to bring up your desktop. Now open Task Manager by either right-clicking the taskbar or by pressing CTRL+ALT+DEL. Now, you will see your game, in this case, Port Royale 3, in the applications list. Right click the game, and click 'Go to Process'. This will take you to the process in the 'Processes' bar. Now, right click the process and then click 'Set Core Affinity'. This should take you to a selection of cores. Since you have a dual core, there will be cores #0 and #1 ticked. Depending on which core you want to use, lets say, #1, untick the Core #0 and then click OK. Now, your game should run on a single core proccesor.