Ensure that you have migrated your web.config file correctly. On the assumption that your applicaiton pool is running in Integrated managed pipeline mode then ensure that the <httpModules> section is correct.
If you change your application pool managed pipeline mode to Classic and all works OK then you have a problem with your web.config file.
Ps a good place to start when checking web.config is for the existence of <add name="InitializationModule" type="EPiServer.Web.InitializationModule, EPiServer" /> in the <httpModules> section.
Try reinstalling IIS and see if you still get the problem.
I seem to have run into to a wall that I can't get passed. I have moved an Episite to IIS 7.5 from IIS 6 and also upgraded from cms 5 to 6. I now get the following exception when I start it. I am certain that there is something missing in my web.config but I can't for the * of it figure out what it is. All help is greatly appreciated.