I also had this problem, i bought the arma2 pack on sale a couple of days ago, added day z then everything on arma2 stopped working, re-installed arma2 and all working again but dunno if i fancy adding dayz again, wasn't sure if i had added the wrong patch etc, day z worked fine but nothing else on arma2 did., shame.. cos i really liked dayz, not hard to ask to be able to play both of them with re-installing is it?