The Microsoft Web Platform Installer (MWPI) is a free download from Microsoft that will help you install all the required components to run ASP.NET web sites on your own computer.
The first step is to download the software from Microsoft:
Double click the file and the MWPI will start up. It might take a minute or two before the interface displays. When it does, click the Web Platform tab. With most of the items you can click to use the defaults, but with the 'Web Server', you should customize it to ensure that the Web Deployment Tool box is checked (see below).
You can then click to install the various components and the tool will ensure you have everything needed to run ASP.NET web apps on your computer.