Algo así, puede ser?
Código:
<html>
<head>
<title> New Document </title>
<script language="JavaScript">
<!--
function act(val){
for (i=0;i<document.forms[0].length;i++){
if ((document.forms[0].elements[ i].type) == "checkbox"){
document.forms[0].elements[ i].checked = false;
}
}
document.forms[0].elements[val].checked = true;
if (val == 3){
document.forms[0].txt_otro.style.visibility="visible";
}
else {
document.forms[0].txt_otro.value="";
document.forms[0].txt_otro.style.visibility="hidden";
}
}
//-->
</script>
</head>
<body bgcolor="#FFFFFF">
<form method=post action="">
<input type="checkbox" value="apart" onClick="act(0);"> Casa <br>
<input type="checkbox" value="quinta" onClick="act(1);"> Apartamento <br>
<input type="checkbox" value="casa" onClick="act(2);"> Quinta <br>
<input type="checkbox" value="otro" onClick="act(3);"> Otro <input type="text" name="txt_otro" style="visibility:hidden;"><br>
</form>
</body>
</html>
<hr noshade size=1><img src="http://www.gograph.com/Images-8712/ClipArt/cat03.gif" height="50" border=0 align="absmiddle"> <font size="2" face="verdana" color="#000000">
bet[/CODE]