How do I make folders on the LAN which can only be opened by authorised people?

You should activate NTFS file security for this purpose.

Follow this step:

1. Logon with administrator previledge

2. Open Windows Explorer -> Right Click the drive contain the data (for example drive D:) and choose Properties. Check on General Tab for File System. It should says: NTFS. If it's not NTFS (e.g: FAT32), go to step 2B else go to step 3

2B. Open DOS Prompt. Type: "CONVERT D: /FS:NTFS" without double-quote.

Example:

C:\> Convert d: /fs:NTFS

Follow the instruction for converting the drive D: from FAT32 to NTFS and restart the computer when necessary.

3. Open Windows Explorer, Go to: Tools -> Folder Options -> View -> UNCHECK [ ] Use simpe file sharing (Recommended) and then click OK

4. Browse the folder contain data that you want to protect. Right click on the folder and choose: Properties -> Security

ADD the account-name who should have access to this files and REMOVE all others including: Administrators, SYSTEM, Creator Owner, etc.

5. Done! Now the file only can be access by the account name stated in NTFS Security.

Warning: Another account-name user with administrators previledges still can take over the ownership of the files. To prevent them to do that do the following extra homework:

1. Go to Start -> Run. Type "SECPOL.MSC" without double-quote

2. Explore: Security Settings -> Local Policies -> User Rights Assignments

3. Find: Take Ownership of files and other objects

4. Double click it. ADD your account-name and REMOVE Others, so no one can take the ownership of files except YOU.

5. Click OK, Close the window and Done!

Create the folder.

2. Right-click > properties > Security tab > Set the permission for the users...Be careful here.

Good luck