Para validar esto al vuelo, es decir impedir que ingresen numeros seria algo asi:
<SCRIPT TYPE="text/javascript">
<!--
// copyright 1999 Idocs, Inc.
http://www.idocs.com
// Distribute this script freely but keep this notice in place
// funcion modificada para que solo acepte string.
// eft0 - forosdelweb.com
function strOnly(myfield, e, dec)
{
var key;
var keychar;
if (window.event)
key = window.event.keyCode;
else if (e)
key = e.which;
else
return true;
keychar = String.fromCharCode(key);
// control keys
if ((key==null) || (key==0) || (key==8) ||
(key==9) || (key==13) || (key==27) )
return true;
else if ((("aAbBcCdDeEfFgGhHiIjJkKlLmMnNñÑoOpPqQrRsStTuUvV wWxXyYzZ").indexOf(keychar) > -1))
return true;
else
return false;
}
//-->
</SCRIPT>
<INPUT NAME="dollar" SIZE=5 MAXLENGTH=5 onKeyPress="return strOnly(this, event)">