how to play cod4

i have installed cod4 in my pc, but every time i try to play cod4 online game it gives me the error of "key in use".
how i could i play cod4 with ptcl servers.please help me...

I played COD:modern warfare a long time back but then i heard about ptcl ppl hosting some server so i installed the game and also updated it to 1.7 version, not just this i did each and everything given on ptcl's gaming lounge but to my utter disappointment nothing worked and finally i uninstalled it and switched on to CS. ab yahan pe bhi sxe-injected 7.8 download kia, i ran it before starting cs but then ptcl server kicked me out every time i tried playing online stating that sxe-inject is not installed.
Syed Adil Asif