http://www.overclock.net/faqs/92457-...core-cpus.html

If you don't care to read it/want a quick summary...

Start up the game
Alt+Tab out of the game
Bring up the Task Manager (ctrl+alt+delete)
Right click the program
Select "Set affinity"
Choose the core you want the program to use

Oh, and Combat Arms isn't a MMORPG....