Jamal,
Is your IIS working correctly?
If you put in http://localhost/ (on your server machine) or http://yourserver.name/ in your browser, what do you get?
Regards
Anthony
One other thing you can try to just verify it's not something that may be left over from a previous install is to go in and remove the C:\Users\{username}\AppData\Roaming\com.esri.ags.AppBuilder3.0 folder.
There is a step for uninstalling App Builder that talks about this. You can find it at http://resources.arcgis.com/en/help/flex-viewer/concepts/index.html#/Installation_guide/01m30000002w....
If this still doesn't work, I would go with what the previous poster stated regarding making sure you can access your IIS web server via localhost.