I didn't go the "disable simple file sharing" route because you
claimed that sometimes you *did* see the Security tab when viewing the
properties of a file. You said (punctuation added) "Sometimes, when I
click Properties for a file, I see a security tab ...".
From
http://support.microsoft.com/kb/307874 (emphasis added):
"If simple file sharing is enabled, the simple file sharing user
interface is displayed INSTEAD OF the Security and Sharing tabs."