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:

IISMan

 

 

 

 

 

 

 

 

 

 

 

Switch to the Service Tab :

WebProp

 

 

 

 

 

 

 

 

 

 

 

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.