Intro
Unfortunately, the Subtext installation process cannot automatically configure a custom 404 (file not found) page for you. But this guide will walk you through this.
The first step is to open up the IIS properties dialog for your site and click on the Custom Errors tab. Sites at hosting providers such as WebHost4Life will have some web-based interface for setting this.
Figure 1: Setting a Custom 404 Page.
Scroll down till you see 404 in the HTTP Error column and double click that item. Select URL for the Message Type field and enter /SystemMessages/FileNotFound.aspx for the URL field.
Figure 1: Setting a Custom 404 Page.
The URL to enter may differ depending on your IIS setup. For example, on my dev server Subtext is running within a virtual application named Subtext.Web so the URL is /Subtext.Web/SystemMessages/FileNotFound.aspx.
Click OK and you are done.