You can enable detailed error messages for your classic ASP site defaulted to Windows/IIS by using a web.config file with the following:
<configuration>
<system.webServer>
<httpErrors errorMode="Detailed" />
</system.webServer>
</configuration>
Note: Detailed errors are an excellent way to troubleshoot your site code in classic ASP because the error messages they generate on your site may contain information that is relevant to the error you are experiencing and will assist you in resolving many coding issues. However, please be mindful when using detailed errors on a production site because the information that is output to the page can also be very revealing about the way your code works and processes information, which is particularly concerning for the security of your site. Therefore we strongly recommend the use of detailed errors for sites that are in development or experiencing problems; conversely we strongly recommend disabling detailed errors while the site is in production and not experiencing any issues.
For more detailed information regarding the syntax and use of please visit:
http://www.iis.net/ConfigReference/system.webServer/httpErrors