How do I make this computer run faster?

My son wants this computer to run faster, quicker. This computer is only a year/two years old ... so I am not willing to replace it. My son plays "World of Warcraft" online and he gets frustrated because others play faster than he can. Is there anything that won't cost me and arm and a leg that I can do to enable this computer to run at lightning speed? All answers will be appreciated. Thanks!