My debugging in Visual Studio (VS2008) stopped working this morning, I think it was as a result of Windows Update on Windows 7 adding some "security updates". I run out-of-the-box VS2008 (SP1) and IIS7 built-in to Windows 7 and I didn't even get the browser, just some non-resizable window with what looked like the html source for a browser error and 500.19. After being annoyed, I decided to try it in a browser with no debugger and got some more useful information but still didn't understand what had changed (Invalid config data because I can't read it type error).
Anyway, eventually I added the IIS_IUSRS group to my code directories with read access (only) and it all started working again.
Not sure whether this permission was not required before or implied or whether one of the updates removed its permissions from everywhere but it was a bit of a pain and not obvious which 'user' the system was trying to access the data with. I assumed the debugger was running as me and I had full permission but obviously somewhere it gets changed from me to IIS_USRS!