You should be prompted with a box like this...

your network user name goes in the username text entry area, your network password goes in the password text entry area,
and jsclark goes in the domain text entry area.

If you do not have a domain text entry area, enter your username as: jsclark\username (i.e. username is the name you use to log on to the network)
note: Windows XP may ask for you to enter it like: jsclark\username) then your password as you would normally.