Tillad kun tal i form felt

<!--
function numbersonly(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;

// numbers
else if ((("0123456789").indexOf(keychar) > -1))
  return true;

// decimal point jump
else if (dec && (keychar == "."))
  {
  myfield.form.elements[dec].focus();
  return false;
  }
else
  return false;
}

//-->
</SCRIPT>

Og her hvordan formularen skal se ud
<input type="text" onKeyPress="return numbersonly(this, event)" name="blabla" size="30">

Glemte lige <SCRIPT TYPE="text/javascript"> i toppen af koden der

Perfekt :)