Debug classic ASP code in Visual Studio 2005

Go to Debug > Attach to Process ...

Select dllhost.exe and click Select... button. Select "Script" checkbox from the dialogue box.

Click OK and ultimately click Attach.

NOTE: Debugging will not work unless your IIS is enabled for .asp debugging. To do that go to your IIS > Home Directory > Configuration > Debugging and check both checkboxes (Enable ASP server-side debugging and Enable ASP client-side debugging)