First, your hard drive must be formatted NTFS for this tab to show up.

Second, if you're running XP Pro, you must open Windows Explorer, go to Tools, Folder Options, View and uncheck Use Simple File Sharing.

Third, if you're running XP Home, Simple File Sharing is enforced by default and cannot be disabled.  You must boot the computer into Safe Mode and log in with the Administrator account, in order to see the Security tab.  A work around for XP Home, so that you don't have to enter Safe Mode, can be found here.