请教一个虚拟域名问题：xxx.com正常，www.xxx.com就不正常了，这是什么原因？

[phoenix.sky]

在WIN7上安装了Apache2.2， 配置了独立虚拟域名，发现：
http://xxx.com, http://xxx/都正常，但http://www.xxx.com就不正常了，这是什么原因引起的？ 谢谢！

httpd.conf配置

ServerName xxx.com:80
<Directory "E:/xxx/xxx">
    Options Indexes FollowSymLinks
    AllowOverride None
    Order allow,deny
    Allow from all
</Directory>
Include conf/extra/httpd-vhosts.conf

httpd-vhosts.conf配置

NameVirtualHost *:80
<VirtualHost *:80>
    DocumentRoot "E:/xxx/xxx"
    ServerName xxx.com 
    ServerAlias www.xxx.com
    ErrorLog "logs/xxx.com-error.log"
    CustomLog "logs/xxx.com-access.log" common
</VirtualHost>

hosts配置

127.0.0.1       xxx
	127.0.0.1       xxx.com
	127.0.0.1       www.xxx.com