To decrease the number of lanes leading to our computers' doors, we can install firewalls.
Some firewalls simply block specified lanes completely.
Some block specified lanes for outside traffic unless we've initiated a conversation on them from the inside. Some firewalls check the contents of the communications traveling on the lanes to see if they are properly formatted and meet our security policy. While simple in concept, real world implementations must deal with the dynamic and eclectic communications needs of those computers as well as the administration needed to keep pace with those needs. A user controlled central firewall is a possible way to get the best of both worlds. Also, personal firewalls, installed on individual machines, can augment the blocking of unwanted communications although they require individual administration on each machine.