DotNetNuke 4.x

DotNetNuke - Rendering PayPal button using a simple aspx page and an iframe

.NET: 

A beginner ASP.NET developer also knows pretty well that each ASP.NET page needs to have a <form runat="server"> tag on the page and that all server controls need to be present inside this tag (although this is not entirely true; you can place server controls outside <form runat="server"> but then they cannot participate in regular ASP.NET functionality like PostBacks, Viewstate etc).

DotNetNuke - Getting files in native DNN folders during installation

I use DotNetNuke extensively for building Enterprise applications for my clients (you might be interested in checking out iClinic, or iMLM etc.). DNN together with ASP.NET & Sql Server put forward extreme power and flexibility in your hands as a Solution Architect, together with great off-the-shelf features, a perfect platform to build upon.

DotNetNuke - Having same usernames across multiple portals on the same installation

You might be well aware that DNN allows you to create multiple Portals based on the same physical installation and the database. It even allows the users to be managed independently across Portals, restricting users to the Portal they registered for.