[color:"orange"]The problem is that he instructs to use the ingame task manager to do this[/color]
You don't need to be able to access the Task Manager from within in the game; the game just has to be running while you are using the Task Manager to set it to use only one CPU. Start the game, alt-tab out (or hit ctrl-esc or the windows key to bring up the start menu, which will minimize the game), then start the Windows Task Manager (hit ctrl-alt-del, or right click on the taskbar and select Task Manager).
Monitoring and Optimizing Apps on Dual-Core and Multiprocessor Systems - briefly describes how to 'Set Affinity' of an application to a specific CPU through the Task Manager or using a couple other utilities.
So long to "Set Affinity"... - mentions another program to set CPU preference, as well as driver updates and an MS hotfix