When Windows XP Home or Pro Editions are in a Workgroup environment, rather then a Domain environment, the Security tab is hidden by default. This is the "Simple File Sharing" configuration, which is rather bland security.

To enable the Security (and Sharing) tabs, and better security control over your files and shares, turn off Simple File Sharing.

To turn off Simple File Sharing, from My Computer, or Windows Explorer:

    Click Tools --> Folder Options --> View
    Uncheck "Use simple file sharing"

Reference:
    http://support.microsoft.com/kb/307874