Hola pato_volador!
con esto podrás hacer lo que pides:
Código:
<html>
<script language="javascript">
function descheck(id) {
for (i=0;i<7;i++) {
mid = 'c' + (i+1)
//alert (mid)
document.getElementById(mid).checked=false
}
document.getElementById(id).checked=true;
}
</script>
<body>
<form id="form1" name="form1" method="post" action="">
<input name="c1" type="checkbox" id="c1" value="checkbox" onclick="descheck('c1')" />
<input name="c2" type="checkbox" id="c2" value="checkbox" onclick="descheck('c2')" />
<input name="c3" type="checkbox" id="c3" value="checkbox" onclick="descheck('c3')" />
<input name="c4" type="checkbox" id="c4" value="checkbox" onclick="descheck('c4')" />
<input name="c5" type="checkbox" id="c5" value="checkbox" onclick="descheck('c5')" />
<input name="c6" type="checkbox" id="c6" value="checkbox" onclick="descheck('c6')" />
<input name="c7" type="checkbox" id="c7" value="checkbox" onclick="descheck('c7')" />
</form>
</body>
</html>
Saludos!