That is most likely styling on the fieldset element in your style.css.  I can't tell for sure from just a picture but that's what I would place my bet on.  You can find out for sure if you have Mozilla Firefox with Firebug installed.  Use Firebug to select to border and you should see if it is, in fact, the fieldset that has the offensive styling.  If it is then you should open your style.css file and do a word search for "fieldset" and then replace whatever styling is there with:

fieldset {
border: 0 none;
margin: 0;
padding: 0;
}

I hope that points you in the right direction,

-Elijah