Like some people already mentioned, you could probably use Gamebooster as it disabled your background processes wich CAN greatly improve your performances.
I myself rarely notice anything when i run Gamebooster tho, but then again i have a Beast computer and i run any game flawlessly.
However, i've seen some people with gamebooster that really DID increase their performance so it would never hurt to try it.
Next i suggest you'd download Tuneup utilities 2011, it's a Program that tweaks your computer to the max ( However inexperienced people can mess up their computers so use it at your own risk. ), I myself use this and it helps me out big time, Tuneup isn't just for better performance you can do ANYTHING with it.
Also i'd suggest you to defrag your computer as it bring all files together so your Harddrive reads it faster.
If your computer still laggs after this... well GET A BETTER COMPUTER <3
EDIT: on Default settings my pc gives 1500+ fps everywhere in Gunz.
but since i underclock my computer for lower games it shoots from 400-900 everywhere, wich is great considering the human eye doesn't see ANY improvement beyond 60 Frames per second.
