IIS will send out server type showing the web server's type, it's kind of trick to some statistics site, the response header like:

Server: Microsoft-IIS/5.0

Server: Microsoft-IIS/6.0

Server: Microsoft-IIS/7.0

In order to remove these header, you can download and install urlscan

http://www.microsoft.com/technet/security/tools/urlscan.mspx

URLScan will add one ISAPI header on the server, it's priority is high

In order to remove, the "Server" header, you need to change

..\inetsrv\urlscan\urlscan.ini

there is one property called RemoveServerHeader, make sure to change it's value to 1

Check the result, but it's better to restart IIS first