Log on as Administrator.
Go to Settings | Control Panel | User Accounts, and click on Guest Account to make sure it is turned on.
Go to Start | Run, type cmd, and hit Enter.
At the prompt, type Net user guest password and hit Enter.
Go back to User Accounts and click on the Guest account.
Note that you can now add a password (see the screen). Do so and reboot.
From now on, any attempt to log on to shared folders from the Guest account will require the Guest password for access-even if the Guest account is turned off!