Network shield is meant to block known internet worms, "NETWORK SHIELD: BLOCKED "DCOM EXPLOIT" - ATTACK FROM 200.217.63.80:135/TCP" basicly means avast has blocked one (dcom exploit) and you are protected/safe. (A decent firewall will block these attacks aswell).
If you would like to stop the messages simply left click the avast "A" icon in your taskbar (next to your system clock) and select "network shield", then click customize, then under settings tab uncheck "show messages" (see below).
--lee
