function check_username()
{
	width = 300;
	height = 200;
	x = (screen.availWidth-width)/2;
	y = (screen.availHeight-height)/2;
	window.open('check_username.php?username='+document.getElementById('user_login').value,'check_username','width='+width+',height='+height+',screenX='+x+',screenY='+y+',left='+x+',top='+y+',resizable=no,statusbar=no,scrollbars=yes');
	return false;
}