There are a few locations where you look at network settings, and change permissions.
Which preference panel you need to open, depends on whether you want settings to apply to a disk, or to a user.
If you are in Windows XP: For a disk, select its Properties, and look for sharing settings. For a specific user, open Control Panels and select Accounts.
You'll have to consult directions for your computer and operating system.
Example, on a PowerMac running OS 10.5...
Highlight a disk.
Press Command-I, or select 'Get Info' from the top menu. A window opens containing disk info and settings.
Look for 'Sharing and Permissions'.
You can change settings for yourself, registered users, and guests.