con esto tu analizalo y modificalo
<script languaje="jscript">
function cheka(form)
{
if(isNaN(form.pc.value))
{
alert("solo numeros")
}
else
{
form.pc.focus()
}
}
</script>
<form>
<input name="pc" type="text" value="">
<input type="button" value="" onclick="cheka(this.form)">
</form>
