How to control NIS2009 local network trust level

Window XP p2p local network workgroup, (no domain). Most computers have shared resources.

How NIS2009 determines local network trust level?

On some PC NIS2009 states that PC is Protected and on other PC it decides that it's shared. Accordingly, some PC's are visible while others aren't.

How to control and set NIS2009 network rules? Whene PC is stated  as Proetected there doesn't seem to be any available option to set it "shared"