To do this, you must be running NTFS as your file system on the drive in question. If your hard disk/partition is not NTFS you will need to convert it. To do this, open a Command Prompt window and enter the following command:
CONVERT X: /FS:NTFS
Where X: is the drive letter you wish to convert. You may also want to see
http://www.aumha.org/a/ntfscvt.htm to ensure that you're getting the optimal conversion.
After this step is completed and you've rebooted the computer, if necessary:
XP PRO: In Windows Explorer, go to Tools, Folder Options, View and uncheck Use Simple File Sharing. Now, when you right click on a drive, folder or file (on an NTFS partition) and select Properties, you'll see a Security tab. Here you can assign or deny permissions based on user name or user group membership.
HOW TO: Set, View, Change, or Remove File and Folder Permissions in Windows XP
http://support.microsoft.com/default.aspx?scid=kb;en-us;308418
HOW TO: Set, View, Change, or Remove Special Permissions for Files and Folders
http://support.microsoft.com/default.aspx?scid=kb;EN-US;308419
HOW TO: Disable Simplified Sharing and Password-Protect a Shared Folder in Windows XP
http://support.microsoft.com/default.aspx?scid=kb;EN-US;307874