I have been using Avast Free for over two years. I also have been using ZoneAlarm Free Firewall the same amount of time. About a year and half ago I upgraded to ZoneAlarm Pro (paid version). During all this time I've had no conflicts between the two. During this time period, I have upgraded OS from 98 to XP to Vista.
regards
johnb
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Vista Home Premium, SP2, 64Bit, Dell 435T, 6GB RAM, IE8, Avast Antivirus 5.0, Spybot S&D, ZoneAlarmPro, Malwarebytes' Anti-Malware, Windows Defender, Windows Malicious Software Removal, SuperAntiSpyware, Spyware Blaster, cCleaner, Glary Utilities, Speccy 1.0.1.132 and Dialup.