Getting an IIS error description

Hi,

I'm using Server.GetLastError() on my custom 500 error page, and it works fine when there's an ASP error.

But when there's an IIS error such as trying to load an ISAPI module that doesnt exist and things like that, I can't seem to get the cause of the error.

Is there a way to do that or does using custom errors cancels your ability to see IIS configuration errors?