Ok, to just check the box then change this:
HTML Code:
<label for="cb_rules_agree" class="full"><input type="checkbox" name="agree" id="cb_rules_agree" tabindex="1" value="1" /> <strong>{vb:rawphrase read_agree_abide_by_rules, {vb:raw vboptions.bbtitle}}</strong></label>
to this:
HTML Code:
<label for="cb_rules_agree" class="full"><input type="checkbox" name="agree" id="cb_rules_agree" tabindex="1" value="1" checked /> <strong>{vb:rawphrase read_agree_abide_by_rules, {vb:raw vboptions.bbtitle}}</strong></label>