Please help me !!

i have poblems with blackops 2,
everytime when i start it it stopps working !
i know my computer insn`t too slow for it.
if it crash i end it with the task manager then steam tells i can´t run it because it is still running (Application running)

please help me !!!

sry for bad english

win 7
old intel i3 (2,3 ghz i think)
4 gb ram
1 gb grafics