<p>The IIS 6.0 is serving my Classical ASP pages in a serial fashion (one at a time)</p>

<p>The #2 request will be handled by the web server only when the #1 request ends. </p>

<p>If the #1 request takes a little longer, the #2 request will have to wait for the #1 ends to starts being handled by IIS.</p>

<p><strong>Is this a missconfiguration in IIS?</strong></p>

<p>The operation system is Windows Server 2003 Standard Edition (Service Pack 2)</p>

Yes, IIS or the site is most likely configured for server-side debugging, which causes all requests to the site to go through a single thread.
To check if this is the case/turn it off:
In the Properties pages for any Web site or Web virtual directory, click the Home Directory or Virtual Directory tab.
Under Application Settings, click Configuration. An application must be created for the button to be active.
Click the Debugging tab.
Un-check the Enable ASP server-side script debugging check box.
(Above steps were copied from the
Debugging ASP Applications in IIS
KB article)