Unline Win98, XP does not limit shares based on password. They base it on username and/or share type. If you are using simple file sharing, it is either read only to everyone or read/write to everyone.
If you want to limit access, you have to do it based on username. First, disable simple file sharing by going to View->Folder Options and uncheck the last option. Next, right click the folder and select sharing. Enable the share, then click the permissions button. Assign permissions to indivitual usernames. You may need to create more than one username on the comptuer depending on how many users have access. Then when you double click the computer in My Network Places, you will be prompted for a username and password. Enter the appropriate password, and that user will see only the shares that he or she has permission to access.