Change checkbox value

Is this how you change checkbox value?
If so, it seems that I cant pass my value for SQL processing once I unchecked the checked box.

Javascipt:

function changevalue()
{
box = eval("document.form1.gopublic");
if (box.checked == false)
{
alert("disallow");
document.form1.gopublic[0].value = "disallow";
}
else
{
alert("allow");
document.form1.gopublic[0].value = "allow";
}
}


HTML

<input name="gopublic" type="checkbox" id="gopublic" value="allow" checked="checked" onclick="changevalue()"/>

un unchecked checkbox is not an active element, thus it's value will not be submit. Use rather a pair of radio buttons, one of them checeked by default.