Can't you guys do the following?
create a bat file where ever the game .exe is and then shortcut to that.
in the bat file throw in start /affinity CPU# game.exe
CPU# is 0 or 1 in most people's cases IIRC. Mind you, this might screw you if you have to login via some intermediate program, but you could probably hunt within the game folder and you'll likely find the front end exe, you could launch that, login and then launch the game exe separately. Also, I hear that this might not be available in windows xp home/pro.