Is your virtual directory configured to ASP.Net, some times when you install the IIS after the .Net framework it will not be configured to .Net. You need to configure the IIS explisitly.
First check the properties of your website ('Default Web Site' in IIS be default), in 'Home Directory' tab, on Click of 'Configuration..' Application Configuration window will open. In that check the application mappings, if you don't find the .aspx item, the IIS is not configured for .Net.
Check - http://www.asp.net/learn/whitepapers/aspnet-and-iis6