I think you mean that you do not want the URL showing in the address bar, is that right?
If so there are several methods.
This method does an urlrewrite...
<system.web>
<urlMappings enabled="true">
<add url="~/home.aspx" mappedUrl="~/index.aspx?pid=01" />
<add url="~/contacts.aspx" mappedUrl="~/index.aspx?pid=02" />
<add url="~/sales.aspx" mappedUrl="~/index.aspx?pid=03" />
</urlMappings>
</system.web>
Obviously you need to adjust the pages and titles to your own needs.