When you configure your IIS webserver and find the following message
Service Unavailable
Then the first solution to the above error is http://support.microsoft.com/default.aspx?id=823552 but if this does not solve the problem try doing the following stuff.
Open the IIS Manager, right click Web Sites and select properties:













Switch to the Service Tab :













Check "Run WWW Service in IIS 5.0 Isolation Mode".
You will be asked for Restart of IIS. Click yes to restart IIS. If you are not asked just restart IIS.