DavidR
3
The minimum I would say is Standard Shield, Web Shield and Internet Mail. The network Shield only monitors those ports commonly used by exploits/worms, etc. and doesn’t really use any resources and it is looking at content, not just inbound activity, but with a router and a firewall you should be OK.
Then it is down to what applications you use as to what shields should be enabled (P2P, Instant Messaging, Outlook/Exchange {MS Outlook not express}).