First of all, you need to enable server-side debugging of the classic ASP script. Do this by running the following commands:
"C:\Program Files (x86)\IIS Express\appcmd.exe" set config "[YOUR_SITE_NAME]" -section:system.webServer/asp /appAllowClientDebug:"True" /appAllowDebugging:"True" /commit:apphost
Where [YOUR_SITE_NAME] is the name of your site. You can find this name by opening:
%USERPROFILE%\Documents\IISExpress\config\applicationhost.config
... and search for your site.
Then start the IIS Express instance from the command line:
"C:\Program Files (x86)\IIS Express\iisexpress.exe" /config:c:\users\kevin\Documents\IISExpress\config\applicationhost.config /site:"[YOUR_SITE_NAME]" /apppool:"Clr2IntegratedAppPool"
Again, [YOUR_SITE_NAME] is the name of your IIS Express website.
Then attach the Visual Studio 2012 debugger and set a breakpoint in the script that you want to debug. Go to your site / script and you should see a breakpoint light up:

Kev
source share