Changing the priority of a program

Hi
I have a game on my computer (if its relevant its AION) and my computer used to be able to run it without much trouble, since then there have been a few updates. I know you can change the priority of a program to change how much cpu power a computer gives to program. When i try to change the priority of this game in task manager (Im using windows 7) it wont allow me to, it simply says "access denied". Is there another way to set the program to high priority temporarily or permanently?

Thanks