ASP.NET and custom 404 page

Try this in web.config (includes 500 error support as well):

<configuration>
    ...
    <system.web>
        ...
        <customErrors mode="On" redirectMode="ResponseRewrite" defaultRedirect="~/Error/500.htm">
            <error statusCode="404" redirect="~/Error/404.htm" />
            <error statusCode="500" redirect="~/Error/500.htm" />
        </customErrors>
        ...
    </system.web>
    ...
    <system.webServer>
        ...
        <httpErrors errorMode="Custom">
            <remove statusCode="404" />
            <error statusCode="404" path="/Error/404.htm" responseMode="ExecuteURL" prefixLanguageFilePath="" />
            <remove statusCode="500" />
            <error statusCode="500" path="/Error/500.htm" responseMode="ExecuteURL" prefixLanguageFilePath="" />
        </httpErrors>
        ...
     </system.webServer>
     ...
</configuration>