How to Force Client like Windows xp and 7 which is joint to domain force to enter user name and password in vpn connection insted of using loged on credential

Hi

 have a TMG server which is install on windows server 2008R2 and is joined to my (2008)domain and I have Windows xp and 7 Client in my domain .

My Client machin is connect to TMG as vpn client in order to access to internet .

my problem is when my client log on to the windows  and want to connect to internet via vpn connection ,whithout entering user name and password they can connect to TMG with windows logged on credential,

I want to force client enter username and password when using vpn connection insted of windows send their loged on credential

thanks in advance