system
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I also run Zone Alarm Pro, WinXp Home, SP2. I have my Network shield activated. It may not need to be but it hasn’t caused me any conflicts that I am aware of. 
Here is some further info: “A new resident protection module was added to avast! 4.5: the Network Shield. It is meant as a protection against known Internet worms/attacks. It analyses all network traffic and scans it for malicious contents. It can be also taken as a lightweight firewall (or more precisely, an IDS (Intrusion Detection System”.