// Uso: onkeydown="if (!isDigit(event)) { return false }"
var ie4up, ns6, ns4, dom;
if ( navigator.appName == "Microsoft Internet Explorer" ) 
   ie4up = parseInt ( navigator.appVersion ) >= 4;
else if ( navigator.appName == "Netscape" ) {
   ns6 = parseInt ( navigator.appVersion ) >= 5;
   ns4 = parseInt ( navigator.appVersion ) < 5;
}

function isDigit ( evt ) {
   var keyCode = evt.which ? evt.which : evt.keyCode;
   digit = !isNaN ( parseInt ( String.fromCharCode ( keyCode ) ) ) || 
   	   // testa o "ponto"
      // ( keyCode == 190 || keyCode == 110 ) || 
      ( keyCode >= 8 && keyCode <= 46 &&
      keyCode !=16 && keyCode !=32 );
   digit = ie4up ? digit || ( keyCode >= 96 && keyCode <=105 ) : digit;
   return ( digit );
}