VPN logon

Hi,
I hope I explain this correctly.
I have a windows nt4.0 domain… I have some clients VPN into the company….When they click there vpn they have to enter their username and password..and they can get on to the network w/no problem…Only on 1 laptop after they connect to the network they get a Enter Network Password which would be Username,password and domain.. I would like this to happen because then they could run a logonscript to map them to some network drives.. My question is why do some get this screen and others do not? Any help would be appreciated…