WIN2008的NLB 只是负载均衡，并没有办法检查网站是否能访问，这时候需要工具来检查网站的可以用性
安装说明：
http://www.iis.net/download/All
http://learn.iis.net/page.aspx/482/install-application-request-routing/
Web Farm Framework
The Microsoft Web Farm Framework (WFF) 2.0 for IIS 7 simplifies the provisioning, scaling, and management of multiple servers for administrators and hosting providers. Administrators can seamlessly provision multiple servers, deploy content to them, and use them to enable elastic scaling. Using the Web Farm Framework, you can also run operations on a server (or a server farm) through a unified interface that also provides up-to-date status of server farm provisioning and health.
检查web存活:
Application Request Routing
Microsoft Application Request Routing Version 1 for IIS 7 is a proxy based routing module that forwards HTTP requests to content servers based on HTTP headers and server variables, and load balance algorithms.
预启动iis加载数据：
Application Warm-Up Module for IIS 7.5 - Beta
IIS Application Warm-Up for IIS 7.5 enables IT Professionals to improve the responsiveness of their Web sites by loading the Web applications before the first request arrives.