Yes, what Mr.X said!
Arimidex prevents the formation of estrogen from testosterone, and Nolva blocks the action of estrogen in the body.
To protect against gyno in the first place, you can run Arimidex through your cycle at .5 - 1mg/day (depending on dose of AAS). This will also eliminate bloat and give you a harder look (at the expense of some sheer size). Be sure to continue arimidex into your PCT if you are using long-acting esters. You can taper off adex post-cycle by decreasing your dose each week until you stop taking it altogether after 4-5 weeks. Tapering off is important to restore normal estrogen production (and normal sex drive) post-cycle.
If you already have estrogen gyno, arimidex after the fact will not help you. Nolva at a reasonably high dose or nolva+femara is the cure.