Windows Server 2008 (IIS7) by default displays non-detailed error messages. This is important for the best security. To enable detailed error messages in your websites you should modify the web.config file in your website's root folder. Adding the "httpErrors" node below within the "configuration/system.webServer" node (Simply create the nodes if they don't exist)
e.g.
<configuration>
<system.webServer>
<httpErrors errorMode="Detailed" />
</system.webServer>
</configuration>
Once added all pages within this website will show detailed error messages when reporting errors. We recommend you remove this setting for the best security on live production websites. Please also see the following URL for more information http://www.iis.net/ConfigReference/system.webServer/httpErrors