Not asking for User name / password

Hi,

I have two Windows NT 4.0 workstation and one win2000 proff in my network

my problem is that, when i start these machines it is not asking for the user name and password it directly loging to the network

i hope while instaling the option was selected that , assuming this particular user is always loggin to the machine

i want these machines to ask for domain password each time i start, can any one help me