Use this for IIS 7
<system.webserver>
<defaultdocument>
<files>
<clear />
<add value="MyLoginPage.aspx" />
</files>
</defaultdocument>
</system.webserver>
In IIS 6 , you can do it using a small hack
<system.web>
<urlMappings>
<add url="~/Default.aspx" mappedUrl="~/YourPage.aspx"/>
</urlMappings>
</system.web>