When you dial a phonebook entry in Dial-Up Networking (DUN), you can use the 'Save Password' option so that your DUN password is cached and you will not need to enter it on successive dial attempts. This key disables that option.
Value Name: DisableSavePassword
Data Type: REG_DWORD
Data: (0=disable, 1=enable)
More Info: http://support.microsoft.com/support/ntserver/serviceware/10140908.asp