Buenas,
Así podría servir:
Código HTML:
Ver original <script type="text/javascript"> window.onload=function()
{
document.getElementById("prueba").onkeypress = function(event)
{
event = event || window.event;
var caracter = event.charCode || event.keyCode;
return (/[a-zA-Z ]/.test(String.fromCharCode(caracter)));
}
}
<input type="text" id="prueba">