Iis – Migrating Application to IIS7.5

I have a web application built in .Net 3.5.
Due to an upgrade to IIS7.5, I have recomplied the application using .net 4.0.
Everything seems to be fine when debugging via the development server however when using IIS, I get the following error when trying to access an aspx file.

HTTP Error 404.17 – Not Found
The requested content appears to be script and will not be served by the static file handler.
Detailed Error Information
Module StaticFileModule
Notification ExecuteRequestHandler
Handler StaticFile
Error Code 0x80070032
Requested URL http://localhost:80/pij/service/Gateway.aspx
Physical Path C:\inetpub\wwwroot\pij\service\Gateway.aspx
Logon Method Anonymous
Logon User Anonymous

Prior to changing the application's app pool to v4.0 Classic I was getting the following.

HTTP Error 500.22 – Internal Server Error
An ASP.NET setting has been detected that does not apply in Integrated managed pipeline mode.

If anyone has any ideas how I can fix this, I'd be most grateful
thanks