function checkname()
{
 if(form.user_name.value=="")
 {
  alert("请输入用户名！");
  return false;
 }
 
 var a=form.user_name.value;
 var b=a.substring(0,1);
 if(!((b>='a'&&b<='z')||(b>='A'&&b<='Z')))
 {
  alert("用户名必须以英文字母开头！且区分大小写！");
	return false;
 } 
 
 checkwindow=window.open("","","width=402,height=232");
 checkwindow.location="checkusername.php?checkname="+form.user_name.value+"";
 checkwindow.moveTo(10,10);
}