.NET is a Microsoft installation. Installation is simple and usually don't give issues. If so, just re-install. Use version 4
Silverlight and Flex are plug-ins that runs within the browser. If the code in webpage has some detection to check if plugin is there, it should automatically initialise an installation from either Microsoft (Silverlight) or Adobe (Flex). You could also do this through a manual installation or have IT run group policy to install on the clients
More related to your problem:
Have you installed the ArcGIS WebAdaptor on the server? And is it working? You should therefor not need to use the 6080 port number in the URL eg. ArcGIS Manger should in your case be http://P2/arcgis/manager (that is if P2 is your ArcGIS server) in stead of http://P2:6080/arcgis/manager
Also, can your network settings relate the IP with the host name (meaning id machine P2 detectable by name)?
My suggestion is not to use localhost but to use the actual machine name(s). That way you can detect if something is not set.
I know with our settings, I have to use the internal server name if I configure an application in app builder but I need to change the server name to the outward facing name in the URL