If you're talking about the free version of Zonealarm, that's just a firewall, but they have an antivirus program as well. Since you already have Avast, you can't run another antivirus program because when two are active at the same time, each thinks the other is a virus causing a conflict and bad results.
i have windows 7 acer mini tower. with avast free . but i was told that i do not need zone alarm as the two does not work together. is it a good idea to have both ?