Cannot remove error using Nz function

[Jerry Maiapu]

Hi there,
I have a log in form that users use to log in to the ACCESS DB.
The username and password are taken from the current logged in user of the windows.

I have put the following as the default value for the
username text box which works fine.
( Takes the current logged in user and punches it into txtusername)

=Environ("UserName")

However, recently for some Workstations the following showed up in the username text box.

 #Name?

ie the default value

=Environ("UserName")

errors out..

How can I solve this? I have tried to put some other text to fabricate the error but its persistent.

=IIf(IsNull(Environ("UserName")),"Enter Username",Environ("UserName"))

=Nz(Environ("UserName"),"Enter UserName")

Can someone help on this..

Thanks

Jerry