Restricting access to specific folders on a Windows server is a function of NTFS permissions.
I suggest that when you set up your users you also place them into security groups.
To restrict access to a folder to a specific user or a group of users you just need to do the following:
Right click on the directory you wan to add permissions to and select properties
Go to the Security Tab
Press the "Edit" Button
Press the "Add" button
Enter in a user's username or the name of a group and press OK
With the user or group highlighted modify the permission as in the screen below
Note permissions are additive. In other words if a user is in multiple groups and has read permissions in the one group and read and write in the other group then that user has read/write permissions. The only exception to this is where they may also be in a third group which is ticked "Deny". If this was the case then "Deny" wins and the user will not have that access.