HTTP Handlers and modules are necessary components for any given web app. While it's conceivable that a developer may never develop a Handler/Module ever, there may be legacy handlers/modules that may exist or third party applications that requires to use them. More information on Handlers and Modules here.

So recently, I ran into an issue where all of my old .ashx and custom handlers blew up when running under IIS. I had upgraded to Vista from XP and being that the web application was dependant upon alot of Handler calls, this proved to be the latest challenge in the life of my Vista tenure.

First off, II7 is not enabled by default with your copy of the Windows Vista. Keeping in mind you'll probably need to research a bit to see if II7 even comes with your flavor of Vista, being that some versions don't have IIS packaged with that build; I've got the ultimate edition, so I'm in the clear (for now).

To enable II7: follow these instructions in this link.

Make sure that you've checked off all or most the following feature-sets from: Internet Information Services > World Wide Web Services > Application Development Features

Enabling those features (.NET Extensibility, ASP, ASP.NET, ISAPI Extensions, ISAPI Filters, SSI) ensures that your instance of II7 has more options when you are ready to configure your website even further (in particularly, the Handlers and Modules Mappings).

After you've gotten that done, follow these instructions to get your handlers and modules working.

HTH