PIX does not support NT domain authentication to the VPN client at this moment.
1 You can use Cisco ACS 3.0 , config Radius redirect the authentication to the NT database.
2 Windows 2000 server or advanced server have IAS (internet authentication service) build-in feature. It supports Radius protocol.
You can use Microsoft Radius with Active Directory user database to authenticate the VPN users for PIX.
Once again, it just use NT user database with Radius authentication, will not same as directly to use NT domain authentication.