A+: Security and Permissions for Files and Folders
Ever since Windows 2000, Windows drives using NTFS have an extra tab, Security, on the file and folder properties sheet, which controls who can access and change a file.
Specific users, or groups of users, can have these rights:
Full Control (incl. delete)
Modify - change a file
Read & Execute - open and run a file
Read - see into a file
Write - replace file contents
List Folder Contents - see what's inside a folder.
The Security tab shows users and groups with rights to a file or foder above, and the bottom section allows you to determine what permissions that person or group has. You can change the permissions in either section.
However, Simple File Sharing in XP will disable the Security tab and therefore is not recommended for business use. You can disable that in Computer or My Computer or in Windows Explorer with Tools | Folder Options | View (tab) | scroll to bottom | clear the checkmark for Use Simple File Sharing.