"I have the ASP.NET debugging checkbox checked"
Is it in IIS in the "Application Config" tab? It doesn't have to be checked in order to debug the ASP .Net application.
1. Check your Web.config, make sure that <compilation debug="true">
2. Right click on on your project, go to "Properties", select "Configuration Properties", hit the "Configuration Manager" button and in the "Active Solution Configuration" drop down select "Debug" option.
3. Try running the project web site under an account with administrative priveleges.