I made a compilation of other information about the Network Shield:
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).
Network Shield protects you from internet worms that spread themselves via various security holes in your system. Typicaly these kind of viruses don't infect files but instead they attack running processes on your PC (either Windows components or some server programs like SQL Server, IIS etc.). These kind of attacks are not easily catched by ordinary antivirus during file or mail scanning. It is not a duplicate work with Standard Shield.
On anything of speed <= 100 mbps the performance penalty should be negligible (unless you have very old hardware). On gigabit/10gigabit networks it may slowi things down considerably though. Few people are using these speeds to access the Internet...
