You can enable detailed error messages for your classic ASP site defaulted to Windows/IIS by using a web.config file with the following code. This is useful in diagnosing issues with a site but for security reasons should be turned off once the site is in production:

<configuration>

  <system.webServer>
    <httpErrors errorMode="Detailed" />
  </system.webServer>
</configuration>

For more information regarding the syntax and use of <httpErrors>, please see this article from iis.net.