XP pro in MS Win Network password issue

Im had my XP pro in a MS Win Network LAN not long ago and i noticed that most comps runnin win2k prompted for username / password before you could access their computers through a workgroup.
For instance, I doubleclick on My Network, then on MS Windows Network, then on a wourgroup and then on an icon representing a win2k comp. Then I am prompted for name / password.
How can I make my XP pro behave in the same manner so that others who try to access my computer throught the workgroup are prompted for username and password? I know how to limit access to individual shared folders and files but I dont want it to be possible to brows which files Im sharing without valid username and password. Thankz in advance!