In IIS 7 we have request filtering available to help with locking down files and directories that you don't want to serve out. This is useful for resources (like configuration) that you want your IIS worker process to have access to, but not serve it out to clients. Things like web.config files fall in to this bucket, and default IIS 7 request filtering configuration denies serving out this extension. However on IIS 6, you don't have request filtering functionality built into the IIS platform. You would need to install stand-alone tools like UrlScan.