<script language="JavaScript">
<!--
function weoCB(chkB){
if(chkB.checked){
txtID.style.display = '';
} else {
txtID.style.display = 'none';
}
}
//-->
</script>
<form name="form">
<input type='checkbox' name='omrostning' value='1' onClick="weoCB(this);">
<br>
<span id="txtID" style="display:none;">Din lilla, söta, urgulliga text här</span>
</form>