You can, in design mode, right-click on the field you want underlined, choose
'Font', and then pick from the 'Underline Style' dropdown menu. This will
underline the form field when empty, and also each letter that is typed in
when it is filled out, without pushing the underline away.
Or, if you just want a line to permanently stay in one place under the field
and be completely unrelated to the fill-ins, you can go to
Insert/Shapes/Lines [this path may be a bit different if you are not in
Vista], choose a line and draw it just a hair under the field, as wide as
you want, and it won't move. [If it does move, right-click the line, go to
'Format AutoShape', and in the Layout tab, choose 'In Front of Text'. It
will always be in the same position, no matter what any text or fields on the
page do.
Lauralee