Let us have a look at how to enable custom error pages to execute a URL in IIS 7 / 7.5.

In IIS 6.0

1. Right click on Web Site (Default Web Site in our case) and click on Properties
2. Under Custom Errors tab, click on the error page you want to customize (in our case 404)
3. Click Edit…, under Message type: select URL and type the URL /custerr/404.htm
   
   
   

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 “Execute a URL on this site” and type /custerr/404.htm
5. Click OK
   
   

Remember, I have a custerr folder in C:\Inetpub\wwwroot folder.

My web.config:

<httpErrors>
<remove statusCode="404" subStatusCode="-1" />
<error statusCode="404" prefixLanguageFilePath="" path="/custerr/404.htm" responseMode="ExecuteURL" />
</httpErrors>

Hope this helps,
Vivek Kumbhar