Ok, you installed IIS server in your Windows Operating System.
Then, I suppose you installed .NET Framework.
This much in enough to run simple .asp page but if you try to run .aspx (i.e. ASP.NET page extension) page, you cannot. The page will not run because ASP.NET is not installed yet in your computer although you have installed .NET Framework. ASP.NET should be installed in order to run .aspx pages.
How to install ASP.NET?
There are some steps you should follow to install ASP.NET in your computer. Well, I am feeling lazy (actually I am a lazy guy, a tortoise :D) to write down the steps here. You can visit the link below that describes very clearly about the steps to install ASP.NET and run .ASPX page.
I myself followed this link to install ASP.NET in my computer for the first time.