if you get the warning from punkbuster cvar fx_sort equals 0 (must be 1), you have to set this variable to 1. i don't know what this variable is used for, but change it, otherwise you will be kicked (and banned for 2 minutes) by punkbuster.
how to change it? you can do it in two ways:
1) open the config file (config_mp.cfg) in your cod2 folder and change the line seta fx_sort "0" to seta fx_sort "1" and save it.
2) open the console ingame with the ~-key (on german keyboards with ^) and type /seta fx_sort 1 in the console. then close the console with the same key you opened it. that's all.