1.Open Visual Studio 2010
2.Open your website in Visual Studio
3.Fire up your web site.
4.In Visual Studio, click ‘Debug’ Menu -> ‘Attach to process’
5.You may need to tick the box labelled ‘Show processes from all users’
6.‘Inetinfo.exe‘ if application protection is low or ”dllhost.exe‘ if application protection is higher. You may get an ‘Attach Security Warning’ popup. If so, continue On. It’s a bit scary at first, but if it’s your own app on your own PC, then you’ll be ok.
7.Add a breakpoint to your code, and navigate to a location where you will hit it.
Source: http://www.jameswiseman.com/blog/2010/10/12/debugging-classic-asp-from-visual-studio-2010/