There was no option in IIS 6.0 to redirect a client browser to a different URL that contains the custom error. Here’s how you do it in IIS 7 / 7.5.

1. Click on the Web Site (Default Web Site in our case)
2. In Features View, double click on Error Pages
3. Select the error page we want to customize (in our case 404)
4. Click Edit..., click “Respond with a 302 redirect” and type the complete URL (e.g.: http://www.dotnetscraps.com/custerr/404.htm)
5. Click OK
   
   

My web.config:

<httpErrors>
<remove statusCode="404" subStatusCode="-1" />
<error statusCode="404" prefixLanguageFilePath="" path=http://www.dotnetscraps.com/custerr/404.htm 
     responseMode="Redirect" />
</httpErrors>

Hope this helps,
Vivek Kumbhar