Authorization of Shared Folder Access

[LeftEyeNine]

Guys, I'm in need of help.

At work, we have defined a folder on a colleague's computer as a writeable-shared one for our use.

Yet we want this folder to be accessed by only that certain department, not any other ones working here. So we need the access process to ask for login/password every time someone tries to browse that folder. "Every time" is the keyword. Currently it does not. You can browse that folder right away.

Looking for your quick help. Thanks in advance.

Edit: All PCs use Windows XP.