Hi AquaHolicz,
You may want to check out this thread, in which the discussion of a locked Hosts file has just recently taken a turn toward also implicating Hostman as the issue. Since others have used a Hosts file with no problems from Norton (other than longer scan times), it looks like some always-running aspect of Hostman might be the culprit.
http://community.norton.com/t5/Norton-Internet-Security-Norton/HOSTS-File/td-p/1108546