Although most home users are typically happy letting anyone at any computer read or modify any file, business users need to restrict access to files with payroll, personnel, and proprietary information. Windows XP and its predecessors, Windows NT and Windows 2000, were primarily designed for business use, so they require usernames and passwords for identification, and have a security system that lets computer owners restrict access to sensitive files on a user-by-user and file-by-file basis on each computer.

Unfortunately, on a Windows workgroup network, there is no centralized list of authorized usernames. This makes maintaining control of who is and isn't permitted to access network files on each computer difficult. Here's why: When you attempt to use a file or printer shared by another computer, Windows sends your username and password to the other computer. In versions of Windows prior to XP

• If the username and password matched a user account already set up on the other computer, Windows used that account's permission settings to determine whether to grant you access to the file.

• If the user information didn't match, Windows prompted you to enter a username and password that the other computer will recognize.

• If you failed to provide a valid password, the remote Windows computer gave you the permissions assigned to the "Guest" account, which was usually disabled or didn't have permission to access the resource you wanted.

The advantage of this system was that it let you determine precisely which users could access specific files and printers. The disadvantage was that it required you to set up identical user accounts for each network user on every computer, and then grant these users permissions to view and modify shared files and folders.

Smaller business and home users found this security setup cumbersome to use and difficult to set up properly. This pushed people into sharing accounts and passwords, and otherwise avoiding good security practices, just to get the network to work. That's a risky approach, so Microsoft gave Windows XP a new feature called Simple File Sharing.

When Simple File Sharing is enabled

• Network users are always given access to shared folders and printers, without being prompted for a username or password. They are automatically granted access to files and folders using the permissions granted to the Guest account, even if Guest is disabled for direct logins.

• The Security properties tab that is normally used to assign per-user permissions to files and printers is not displayed, even for files that are stored on a drive formatted with the NTFS file system. Files stored in directories in a user's profile folder (My Documents, for example) are automatically set up to permit access only by the owner, and files stored elsewhere are set up to permit access by anyone.

• Windows automatically assigns appropriate security permissions to folders and printers when you share them. If you check Allow Network Users to Change My Files, all network users can read, write, rename, or delete the contents of the shared folder. If you don't check this option, network users can view but not modify the contents.

The result is that, with Simple File Sharing in effect, anyone who connects to your network will have access to all files, folders, and printers shared from a Windows XP computer, with no security enforcement of any kind. This has the advantage of eliminating all worries about having to manage accounts and passwords on multiple computers, but it does mean that you have to keep in mind these points:

• The "Shared Documents" folder that appears under My Computer is automatically set up as a shared folder, the idea being that any files you place in it will be available not only to other users on your computer, but to other users anywhere on your network.

• You don't get to pick and choose who gets access and who doesn't. Everyone gets access to every shared resource.

• If you have an unsecured wireless network with no WEP or WPA security key, anyone driving by your home or office will not only be able to connect to your network, they'll be able to see and/or modify your shared files.

In the end, it's a reasonable trade-off, as long as keep in mind the fact that all shared files and folders are available to anyone who can connect to your network. You should also keep in mind that

• Simple File Sharing is always used on XP Home Edition, and cannot be disabled. This means that anyone can use any resource shared by a computer running XP Home Edition.

• Simple File Sharing is optional on XP Professional, when the computer is part of a workgroup network. It's enabled by default when Windows is installed, but you can disable it if you want to use the user-level security on files and/or shared resources.

• Simple File Sharing is always disabled on an XP Professional computer when it's joined to a domain network. User-level security is always used in this case.

• Simple File Sharing applies only to the resources shared by the computer running Windows XP. If you use XP to used folders shared by a computer running some other operating system, Windows Me or Mac OS X, for example, that operating system's security system will be used.

On an XP Professional computer that is not a member of a domain network, the Simple File Sharing feature can be disabled from the Tools, Folder Options, View tab in any Windows Explorer window. You must be logged on as a Computer Administrator to change the setting.

Also, if Simple File Sharing is disabled, your computer will display different dialog boxes when you go to share a folder, and you'll have access to the Security properties page on folders and printers.