404 re-directs not working for non-CFM extensions

Here's my situation ... when someone goes to an incorrect URL under our domain that ends with a CFM extension, they are correctly re-directed to our 404 page; however, if the user types a URL, or clicks on a link, that ends in any extension other than CFM (i.e. HTML, PHP, ASPX, etc.) they are presented with a message that states "The system cannot find the file specified" instead of our 404 page.

We are using Coldfusion MX7 on IIS6, Windows 2K3. Personally, I am more of a front-end designer/developer than anything else, so my actual  knowledge of IIS and Coldfusion administration is minimal at best. My supervisor, who is equally baffled by this, does have a better understanding of IIS but is equally novice at CF admin. (We inherited this system so are forced to learn as we go.)

Any assistance would be very much appreciated. Thank you!

Cheers!

James