First thing you need to do is turn off 'Enable Friendly HTTP Error Messages in your IE settings.' If you've alredy done this, then you need to turn off IIS's custom error messages so that you can get something more detailed. You can do this from the IIS7 Admin tool by running Start>Run>inetmgr.exe, selecting your website/application/virtual directory in the left-hand tree view, clicking on the Error Pages icon, clicking Edit Feature Settings action, and then selecting Detailed Errors. Then try loading your page again and you should get some more detailed information to help you troubleshoot.
Another thing you MAY need to enable is parent paths. By default they were disabled starting with IIS 6, but a lot of classic ASP apps use them so you may have them enabled. To enabled them on IIS 7, you double click on the ASP section of your website within IIS, and set Enable Parent Paths to true.
Based on everything else it sounds like you should have what you need in order to move the app to IIS 7.