the operation could not be completed access is denied

I try to change the priority of warcraft 3 but it tells me "the operation could not be completed access is denied". Yes I am the only user and administator on the pc so how can I make the warcraft 3 priority higher? I play the game sometimes and when I do I lag in this game called green td because of to many monsters or something. I know its not my pc because I can play GTA IV on high/highest on all settings at 1920x1080 resolution with a solid 25 fps with normal priority. Anyways here my specs:

AMD Phenom II X2 511 3.4Ghz
5gigs of ddr3 ram
Nvidia Geforce 9600 GSO verto 768MB GDDR3
750GB HD with 435GB free

Green td is the only game I lag at so I wanna try changing the priority but I can't so any help would be great.