It most likely wont work at all if it doesnt meet the minimum requirements. Either that or it will work and it will just crash. If its the "recommended requirements" your not meeting, but it does meet the "minimum requirements" it will work, but the gameplay will be really low FPS and probably fairly unplayable.
In short - if you dont meet the minimum requirements - i wouldnt even bother.
Agree with the above, best case scenario when running a game on a system below minimum requirements is that the game will run, but you will get unacceptably low framerates, even on the lowest graphical settings. Unless you are able to overclock your CPU up to 2.4 GHz, don't bother trying to run Black Ops.