changing checkbox label in checkbox generated in vb.net

I am trying to have a checkbox say something different when it is checked. I could use an onclick event to do this but the problem lies in the way vb.net is rendering the checkbox. I am specifically having problems getting access to the label. With vb.net it is generating the checkbox as a checkbox with corresponding label linked by a for:

<input id="_ctl25_checkBoxLine2" type="checkbox" name="_ctl25:checkBoxLine2"/><label for="_ctl25_checkBoxLine2n">Line2n</label>

How do I get access to the label if it doesn't have an id? So getElementById won't work. Is there someway to use the label's for= attribute?

Any suggestions would be appreciated. thanks.