<form> <input type="text" name="txt1" onkeydown="checkKey(this)" /> <br /></form><script type="text/javascript">function checkKey(obj)
{
if((event.keyCode >= 48 && event.keyCode <= 57) || event.keyCode == 8 //Backspace || event.keyCode == 9 //Tab || event.keyCode == 35 //End || event.keyCode == 36 //Home || event.keyCode == 37 //Left || event.keyCode == 39 //Right || event.keyCode == 190 //. ) 
{
if(event.keyCode == 190) { if(obj.value.indexOf('.') >= 0) event.returnValue = false; else event.returnValue = true;
} else event.returnValue = true; } else { event.returnValue = false; }
}</script>